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UNIVERSITY
JJBRARX
Cornell University Library
QC 6.E35 1920
Relativity; the special and gener^^^^^^^^^
3 1924 011 804 774
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RELATIVITY
TBE SPECIAL AND GENERAL THEOBY
1 «, S'
C^ ^1
fMtr
_l
RELATIVITY
THE SPECIAL AND GENERAL THEORY
BY
ALBERT EINSTEIN, Ph.D.
FBOFSasOB 07 FBTSICS IH THS tTHITIlBBITT OF BEBLIH
TRANSLATED BY
EGBERT W. LAWSON, M.Sc.
UHIVEUSITT OF SHEFFIELD
NEW YORK
HENRY HOLT AND COMPANY
1921
COFYBIGHT, 19S80
BT
HENBY HOLT AND COMPANY
PREFACE
THE present book is intended, as far as pos-
sible, to give an exact insight into the
theory of Relativity to those readers who,
from a general scientific and philosophical point
of view, are interested in the theory, but who are
not conversant with the mathematical apparatus '
of theoretical physics. The work presimies a
standard of education corresponding to that of a
university matriculation examination, and, de-
spite the shortness of the book, a fair amount
of patience and force of will on the part of the
reader. The author has spared himself no pains
in his endeavour to present the main ideas in the
simplest and most intelligible form, and on the
' The mathematical fimdaments of the special theory of rela-
tivity are to be found in the original papers of H. A. Lorentz, A.
Einstein, H. Minkowski' published under the title Das RelativiUUs-
prinzip (The Principle of Relativity) in B. G, Teubner's collection
of monographs Fortschritte der mathemaiischen W issenschaften (Ad-
vances in the Mathematical Sciences), also in M. Lane's exhaustive
book Das RdativiUttsprinzip — published by Friedr. Vieweg & Son,
Braunschweig. The general theory of relativity, together with the
necessary parts of the theory of invariants, is dealt with in the
author's book Die Grundlagen der allgemeinen RelativiUltstheorie
(The Foundations of the General Theory of Relativity) — Joh.
Ambr. Barth, 1916; this book assumes some familiarity with the
special theory of relativity.
V
vi RELATIVITY
whole, in the sequence and connection in which
they actually originated. In the interest of
clearness, it appeared to me inevitable that I
should repeat myself frequently, without paying
the sHghtest attention to the elegance of the
presentation. I adhered scrupulously to the
precept of that brilliant theoretical physicist,
L. Boltzmann, according to whom matters of
elegance ought to be left to the tailor and to the
cobbler. I make no pretence of having with-
held from the reader diffictdties which are in-
herent to the subject. On the other hand, I have
purposely treated the empirical physical founda-
tions of the theory in a "step-motherly" fashion,
so that readers imfamiliar with physics may not
feel like the wanderer who was imable to see the
forest for trees. May the book bring some one
a few happy hours of suggestive thought!
A. EINSTEIN
December, igi6
NOTE TO THE THIRD EDITION
IN the present year (1918) an excellent and
detailed manual on the general theory of
relativity, written by H. Weyl, was pub-
Ushed by the firm Julius Springer (Berlin). This
book, entitled Raum — Zeit — Materie (Space —
Time — Matter), may be warmly recommended
to mathematicians and physicists.
BIOGRAPHICAL NOTE
ALBERT EINSTEIN is the son of German-
Jewish parents. He was bom in 1879 ^
the town of Uhn, Wiirtemberg, Germany.
His schooldays were spent in Munich, where he
attended the Gymnasium until his sijrteenth year.
After leaving school at Munich, he accompanied his
parents to MUan, whence he proceeded to Switzer-
land six months later to continue his studies.
From 1896 to 1900 Albert Einstein studied
mathematics and physics at the Technical High
School in Zurich, as he intended becoming a
secondary school (Gymnasium) teacher. For
some time afterwards he was a private tutor,
and having meanwhile become naturalised, he
obtained a post as engineer in the Swiss Patent
Office in 1902, which position he occupied tiU 1909.
The main ideas involved in the most important
of Einstein's theories date back to this period.
Amongst these may be mentioned: The Special
Theory of Relativity, Inertia of Energy, Theory of
the Brownian Movement, and the Quantum-Law
of the Emission and Absorption of Light (1905).
These were followed some years later by the
vii
viii RELATIVITY
Theory of the Specific Heat of Solid Bodies, and the
fundamental idea of the General Theory of Relativity.
During the interval 1909 to 191 1 he occupied
the post of Professor Extraordinarius at the
University of Zurich, afterwards being appointed
to the University of Prague, Bohemia, where he
remained as Professor Ordinarius until 1912.
In the latter year Professor Einstein accepted a
similar chair at the Polytechnikum, Zurich, and
continued his activities there until 1914, when
he received a call to the Prussian Academy of
Science, Berlin, as successor to Van't Hoff.
Professor Einstein is able to devote himself
freely to his studies at the Berlin Academy, and
it was here that he succeeded in completing his
work on the General Theory of Relativity (1915-
17). Professor Einstein also lectures on various
special branches of physics at the University of
Berlin, and, in addition, he is Director of the
Institute for Physical Research of the Kaiser
Wilhelm Gesellschaft.
f Professor Einstein has been twice married.
His first wife, whom he married at Berne in 1903,
was a fellow-student from Serbia. There were
two sons of this marriage, both of whom are liv-
ing in Zurich, the elder being sixteen years of age.
Recently Professor Einstein married a widowed
cousin, with whom he is now living in Berlin.
R. W. L.
TRANSLATOR'S NOTE
IN presenting this translation to the English-
reading public, it is hardly necessary for me
to enlarge on the Author's prefatory remarks,
except to draw attention to those additions to the
book which do not appear in the original.
At my request. Professor Einstein kindly sup-
plied me with a portrait of himself, by one of
Germany's most celebrated artists. Appendix III,
on "The Experimental Confirmation of the Gen-
eral Theory of Relativity," has been written
specially for this translation. Apart from these
valuable additions to the book, I have included
a biographical note on the Author, and, at the
end of the book, an Index and a hst of Enghsh
references to the subject. This list, which is
more suggestive than exhaustive, is intended as
a guide to those readers who wish to pursue the
subject farther.
I desire to tender my best thanks to my col-
leagues Professor S. R. Mihier, D.Sc, and Mr.
W. E. Curtis, A.R.C.SC., F.R.A.S., also to my
friend Dr. Arthur Hohnes, A.R.C.Sc., F.G.S.,
X RELATIVITY
of the Imperial College, for their kindness in
reading through the manuscript, for helpful
criticism, and for numerous suggestions. I owe
an expression of thanks also to Messrs. Methuen
for their ready coimsel and advice, and for the
care they have bestowed on the work during the
course of its publication.
ROBERT W. LAWSON
The Physics Labosatory
The University of Sheffieid
June 12, 1920
CONTENTS
PART I
THE SPECIAL THEORY OF RELATIVITY,
PAGE
I. Physical Meaning of Geometrical Propo-
sitions I
n. The System of Co-ordinates 5
III. Space and Time in Classical Mechanics . 9
' rv. The GalUeian System of Co-ordinates . . 12
■VV. The Prmciple of Relativity (in the Re-
stricted Sense) 14
VI. The Theorem of the Addition of Velocities
employed in Classical Mechanics ... 19
VII. The Apparent IncompatibiUty of the Law
of Propagation of Light with the Prin-
ciple of Relativity 21
VIII. On the Idea of Time in Physics .... 25
DC. The Relativity of Simultaneity 30
X. On the Relativity of the Conception of
Distance 34
XI. The Lorentz Transformation 36
XII. The Behaviour of Measuring-Rods and
Clocks in Motion 42
3d
xii RELATIVITY
PAGE
Xm. Theorem of the Addition of Velocities.
The Experiment of Fizeau ...... 45
XrV. The Heuristic Value of the Theory of
Relativity 50
XV. General Results of the Theory 52
XVI. Experience and the Special Theory of
Relativty . . ; 58
XVn. Minkowski's Four-dimensional Space . . 65
PART n
THE GENERAL THEORY OF RELATIYITY
XVIII. Special and General Principle of Relativity 69
XIX. The Gravitational Field 74
XX. The Equality of Inertial and Gravitational
Mass as an Argument for the General
Postulate of Relativity 78
XXI. In what Respects are the Foundations of
Classical Mechanics and of the Special
Theory of Relativity imsatisfactory? . 84
XXn. A Few Inferences from the General Prin-
ciple of Relativity 87
XXIII. Behaviour of Clocks and Measuring-Rods
on a Rotating Body of Reference ... 93
XXIV. Euclidean and Non-Euclidean Continuum 98
XXV. Gaussian Co-ordinates 103
XXVI. The Space-time Continuum of the Special
Theory of Relativity considered as a
Euclidean Continuum 108
CONTENTS xiii
PAGE
XXVn. The Space-time Continuum of the General
Theory of Relativity is not a Euclidean
Continuum iii
XXVIII. Exact Formulation of the General Prin-
ciple of Relativity 115
XXIX. The Solution of the Problem of Gravi-
tation on the Basis of the General
Principle of Relativity 119
PART ni
CONSIDERATIONS ON THE UNIVERSE
AS A WHOLE
XXX. Cosmological Difficulties of Newton's
Theory 125
XXXI. The Possibility of a "Fmite" and yet
"Unbounded" Universe 128
XXXn. The Structure of Space according to the
General Theory of Relativity .... 135
APPENDICES ^
I, Simple Derivation of the Lorentz Transfor-
mation 139
n. Minkowski's Four-dimensional Space (" World ")
[Supplementary to Section XVII.J .... 146
ni. The Experimental Confirmatiop of the General
Theory of Relativity 148
(a) Motion of the PeriheUon of Mercury . . 150
(6) Deflection of Light by a Gravitational
Field 152
(c) Displacement of Spectral Lines towards
the Red iSS
BrBLIOGRAPHY l6l
Index 165
RELATIVITY
PART I
THE SPECIAL THEORY OF RELATIVITY
PHYSICAL MEANING OF GEOMETRICAL
PROPOSITIONS
IN your schooldays most of you who read this
book made acquaintance ■with the noble build-
ing of Euclid's geometry, and you remember
— perhaps with more respect than love — the
magnificent structure, on the lofty staircase of
which you were chased about for imcounted
hours by conscientious teachers. By reason of
your past experience, you would certainly regard
every one with disdain who should pronounce
even the most out-of-the-way proposition of this
science to be untrue. But perhaps this feeling of
proud certainty would leave you immediately if
some one were to ask you: " What, then, do you
mean by the assertion that these propositions are
true? " Let us proceed to give this question a
little consideration.
Geometry sets out from certain conceptions such
as " plane," " point," and " straight line," with
1
8 SPECIAL THEORY OF RELATIVITY
which we are able to associate more or less defi-
nite ideas, and from certain simple propositions
(axioms) which, in virtue of these ideas, we are
inclined to accept as " true." Then, on the basis
of a logical process, the justification of which we
feel ourselves compelled to admit, all remaining
propositions are shown to foUow from those axioms,
i.e. they are proven. A proposition is then correct
(" true ") when it has been derived in the recog-
nised manner from the axioms. The question of
the " truth " of the individual geometrical propo-
sitions is thus reduced to one of the " truth " of
the axioms. Now it has long been known that
the last question is not only imanswerable by the
methods of geometry, but that it is in itself en-
tirely without meaning. We cannot ask whether
it is true that only one straight line goes through
two points. We can only say that Euclidean ge-
ometry deals with things called " straight lines,"
to e^ch of which is ascribed the property of being
imiquely determiaed by two points situated on it.
The concept " true " does not tally with the
assertions of pure geometry, because by the word
" true " we are eventually in the habit of desig-
nating always the correspondence with a " real "
object; geometry, however, is not concerned with
the relation of the ideas involved in it to objects
of experience, but only with the logical connection
of these ideas among themselves.
GEOMETRICAL PROPOSITIONS 3
It is not difficult to imderstand why, in spite of
this, we feel constrained, to call the propositions of
geometry "true." Geometrical ideas correspond
to more or less exact objects in nature, and these
last are xmdoubtedly the exclusive cause of the
genesis of those ideas. Geometry ought to refrain
from such a course, in order to give to its structure
the largest possible logical unity. The practice,
for example, of seeing in a "distance" two marked
positions on a practically rigid body is something
which is lodged deeply in our habit of thought.
We are accustomed further to regard three points
as being situated on a straight line, if their ap-
parent positions can be made to coincide for ob-
servation with one eye, imder suitable choice of
our place of observation.
If, in pursuance of our habit of thought, we now
supplement the propositions of EucUdean geometry
by the single proposition that two points on a
practically rigid body always correspond to the
same distance (line-interval), independently of
any changes in position to which we may subject
the body, the propositions of EucUdean geometry
then resolve themselves into propositions on the
possible relative position of practically rigid bodies.*
' It follows that a natiiral object is associated also with a straight
line. Three points A, B and C on a rigid body thus lie in a straight
line when, the points A and C being given, B is chosen such that the
sum of the distances AB and BC is as short as possible. This in-
complete suggestion will suffice for our present purpose.
4 SPECIAL THEORY OP RELATIVITY
Geometry which has been supplemented in
this way is then to be treated as a branch of
physics. We can now legitimately ask as to the
"truth" of geometrical propositions interpreted
in this way, since we are justified in asking whether
these propositions are satisfied for those real things
we have associated with the geometrical ideas. In
less exact terms we can express this by saying
that by the "truth" of a geometrical proposition
in this sense we understand its vaUdity for a con-
struction with ruler and compasses.
Of course the conviction of the "truth" of geo-
metrical propositions in this sense is founded
exclusively on rather incomplete experience. For
the present we shall assume the "truth" of the
geometrical propositions, then at a later stage
(in the general theory of relativity) we shall see
that this "truth" is limited, and we shall consider
the extent of its limitation.
n
THE SYSTEM OF CO-ORDINATES
ON the basis of the physical interpretation of
distance which has been indicated, we are
also in a position to estabUsh the distance
between two points on a rigid body by means of
measurements. For this p;rrpose we require a
"distance" (rod S) which is to be used once and
for aU, and which we employ as a standard measure.
If, now, A and B are two points on a rigid body,
we can construct the line joining them according
to the rules of geometry; then, starting from A,
we can mark off the distance S time after time
untU we reach B. The mmiber of these operations
required is the nimierical measure of the distance
AB. This is the basis of aU measurement of
length.!
Every description of the scene of an event or of
the position of an object in space is based on the
specification of the point on a rigid body (body of
reference) with which that event or object coin-
' Here we have assumed that there is nothing left over, i.e. that
the measurement gives a whole number. This difiSculty is got over
by the use of divided measuring-rods, the introduction of which
does not demand any fundamentally new method.
5
6 SPECIAL THEORY OF RELATIVITY
cides. This applies not only to scientific descrip-
tion, but also to everyday life. If I analyse the
place specification "Trafalgar Square, London," ^
I arrive at the following result. The earth is the
rigid body to. which the specification of place
refers; "Trafalgar Square, London" is a well-
defined point, to which a name has been assigned,
and with which the event coincides in space.*
This primitive method of place specification
deals only with places on the surface of rigid bodies,
and is dependent on the existence of points on
this surface which are distingiiishable from each
other. But we can free ourselves from both of
these limitations without altering the nature of
our specification of position. If, for instance, a
cloud is hovering over Trafalgar Square, then we
can determine its position relative to the svirface
of the earth by erecting a pole perpendicularly on
the Square, so that it reaches the cloud. The
length of the pole measured with the standard
measuriQg-rod, combined with the specification of
the position of the foot of the pole, supplies us
with a complete place specification. On the basis
'■ I have chosen this as being more familiar to the English reader
than the "Potsdamer Platr, Berlin," which is referred to in the
original. (R. W. L.)
' It is not necessary here to investigate further the significance
of the expression "coiacidence in space." This conception is suf-
ficiently obvious to ensure that differences of opinion are scarcely
likely to arise as to its applicability in practice.
THE SYSTEM OF CO-ORDINATES 7
of this illustration, we are able to see the manner
in which a refinement of the conception of position
has been developed.
(a) We imagine the rigid body, to which the
place specification is referred, supplemented in
such a manner that the object whose position we
require is reached by the completed rigid body.
(6) In locating the position of the object, we
make use of a number (here the length of the pole
measured with the measuring-rod) instead of
designated points of reference.
(c) We speak of the height of the cloud even
when the pole which reaches the cloud has not
been erected. By means of optical observations
of the cloud from diEEerent positions on the
ground, and taking into accoimt the properties of
the propagation of hght, we determine the length
of the pole we should have required in order to
reach the cloud.
From this consideration we see that it will be
advantageous if, in the description of position, it
should be possible by means of numerical measures
to make ourselves independent of the existence of
marked positions (possessing names) on the rigid
body of reference. In the physics of measurement
this is attained by the application of the Cartesian
system of co-ordinates.
This consists of three plane surfaces perpendicu-
lar to each other and rigidly attached to a rigid
8 SPECIAL THEORY OF RELATIVITY
body. Referred to a system of co-ordinates, the
scene of any event will be detennined (for the
main part) by the specification of the lengths of
the three perpendiculars or co-ordinates (x, y, z)
which can be dropped from the scene of the event
to those three plane surfaces. The lengths of
these three perpendiculars can be determined by
a series of manipulations with rigid measuring-
rods performed according to the rules and methods
laid down by Euclidean geometry.
In practice, the rigid surfaces which constitute
the system of co-ordinates are generally not
available; furthermore, the magnitudes of the co-
ordinates are not actually determined by con-
structions with rigid rods, but by indirect means.
If the results of physics and astronomy are to
maintain their clearness, the physical meaning of
specifications of position must always be sought
in accordance with the above considerations.^
We thus obtain the following result: Every
description of events in space involves the use of
a rigid body to which such events have to be
referred. The resulting relationship takes for
granted that the laws of EucHdean geometry hold
for "distances," the "distance" being represented
physically by means of the convention of two
marks on a rigid body.
^ A refinement and modification of these views does not become
necessary imtil we come to deal with the graeral theoty of relativity,
treated in the second part of this book.
m
SPACE AND TIME IN CLASSICAL MECHANICS
"'TT^HE purpose of mechanics is to describe how
I bodies change their position in space with
time." I should load my conscience with
grave sins against the sacred spirit of lucidity
were I to formulate the aims of mechanics in this
way, without serious reflection and detailed ex-
planations. Let us proceed to disclose these sins.
It is not clear what is to be imderstood here by
"position" and "space." I stand at the window
of a railway carriage which is travelling uniformly,
and drop a stone on the embankment, without
throwing it. Then, disregarding the influence of
the air resistance, I see the stone descend in a
straight line. A pedestrian who observes the mis-
deed from the footpath notices that the stone
falls to earth in a paraboUc curve. I now ask:
Do the "positions" traversed by the stone He "in
reahty" on a straight line or on a parabola?
Moreover, what is meant here by motion "in
space" ? From the considerations of the previous
section the answer is self-evident. In the first
place, we entirely shun the vague word "space,"
10 SPECIAL THEORY OF RELATIVITY
of which, we must honestly acknowledge, we can-
not form the slightest conception, and we replace
it by "motion relative to a practically rigid body
of reference," The positions relative to the body
of reference (railway carriage or embankment)
have already been defined in detail in the preced-
ing section. If instead of "body of reference"
we insert "system of co-ordinates," which is a
useful idea for mathematical description, we are
in a position to say: The stone traverses a straight
line relative to a system of co-ordinates rigidly
attached to the carriage, but relative to a system
of co-ordinates rigidly attached to the ground
(embankment) it describes a parabola. With ihej
aid of this example it is clearly seen that there is
no such thing as an independently existing tra-
jectory (ht. "path-curve" 0> but only a trajectory
relative to a particular body of reference.
In order to have a complete description of the
motion, we must specify how the body alters its
position with time; i.e. for every point on the
trajectory it must be stated at what time the
body is situated there. These data must be
supplemented by such a definition of time that,
in virtue of this definition, these time-values can
be regarded essentially as magnitudes (results of
measurements) capable of observation. If we
take our stand on the groimd of classical me-
' That is, a curve along which the body moves.
SPACE AND TIME 11
chanics, we can satisfy this requirement for our
illustration in the following manner. We imagine
two clocks of identical construction; the man at
the railway-carriage window is holding one of
them, and the man on the footpath the other.
Each of the observers determines the position on
his own reference-body occupied by the stone at
each tick of the clock he is holding in his hand.
In this connection we have not taken account of
the inaccuracy involved by the finiteness of the
velocity of propagation of light. With this and
with a second difficulty prevailing here we shall
have to deal in detail later.
IV
THE GALILEIAN SYSTEM OF
CO-ORDINATES
AS is well known, the fundamental law of the
mechanics of Galilei-Newton, which is
known as the law of inertia, can be stated
thus: A body removed sufficiently far from other
bodies continues in a state of rest or of imiform
motion in a straight line. This law not only says
something about the motion of the bodies, but it
also indicates the reference-bodies or systems of
co-ordinates, permissible m mechanics, which can
be used in mechanical description. The visible
fixed stars are bodies for which the law of inertia
certainly holds to a high degree of approximation.
Now if we use a system of co-ordinates which is
rigidly attached to the earth, then, relative to
this system, every fixed star describes a circle of
immense radius in the course of an astronomical
day, a result which is opposed to the statement of
the law of inertia. So that if we adhere to this law
we must refer these motions only to systems of co-
ordinates relative to which the fixed stars do not
move in a circle. A system of co-ordinates of
le
THE GALILEIAN SYSTEM 13
which the state of motion is such that the law of
inertia holds relative to it is called a "Galileian
system of co-ordinates." The laws of the me-
chanics of Galilei-Newton can be regarded as valid
only for a Galileian system of co-ordinates.
THE PRINCIPLE OF RELATIVITY (IN THE
RESTRICTED SENSE)
IN order to attain the greatest possible clear-
ness, let us return to our example of the rail-
way carriage supposed to be travelling
uniformly. We call its motion a uniform transla-
tion ("uniform" because it is of constant velocity
and direction, "translation" because although
the carriage changes its position relative to the
embankment yet it does not rotate in so doing).
Let us imagine a raven flying through the air in
such a manner that its motion, as observed from
the embankment, is vmiform and in a straight line.
If we were to observe the flying raven from the
moving railway carriage, we should find that the
motion of the raven would be one of different veloc-
ity and direction, but that it would stiU be uni-
form and in a straight line. Expressed in an
abstract manner we may say: If a mass m is
moving uniformly in a straight line with respect
to a co-ordinate system K, then it will also be
moving vmiformly and in a straight Une relative
to a second co-ordinate system K', provided that
14
THE PRINCIPLE OF RELATIVITY IS
the latter is executing a uniform translatory
motion with respect to K. In accordance with the
discussion contained in the preceding section, it
follows that:
If X is a GaUleian co-ordinate system, then
every other co-ordinate system K' is a Galileian
one, when, in relation to K, it is in a condition of
uniform motion of translation. Relative to K'
the mechanical laws of Galilei-Newton hold good
exactly as they do with respect to K.
We advance a step farther in our generalisation
when we express the tenet thus: If, relative to
K, K' is a uniformly moving co-ordinate system
devoid of rotation, then natural phenomena nm
their course with respect to K' according to
exactly the same general laws as with respect to
K. This statement is called the principle of
relativity (in the restricted sense).
As long as one was convinced that all natural
phenomena were capable of representation with
the help of classical mechanics, there was no need
to doubt the vaUdity of this principle of relativity.
But in view of the more recent development of
electrodynamics and optics it became more and
more evident that classical mechanics affords an
insufficient foundation for the physical description
of all natural phenomena. At this juncture the
question of the vaHdity of the principle of relativity
became ripe for discussion, and it did not appear
16 SPECIAL THEORY OF RELATIVITY
impossible that the answer to this question might
be in the negative.
Nevertheless, there are two general facts which
at the outset speak very much in favour of the
validity of the principle of relativity. Even
though classical mechanics does not supply us
with a sufficiently broad basis for the theoretical
presentation of all physical phenomena, still we
must grant it a considerable measure of "truth,"
since it supplies us with the actual motions of the
heavenly bodies with a delicacy of detail little
short of wonderful. The principle of relativity
must therefore apply with great accuracy in the
domain of mechanics. But that a principle of
such broad generahty should hold with such
exactness in one domain of phenomena, and yet
should be invaUd for another, is a priori not very
probable.
We now proceed to the second argument, to
which, moreover, we shall return later. If the
principle of relativity (in the restricted sense)
does not hold, then the GalUeian co-ordinate
systems K, K', K", etc., which are moving imi-
formly relative to each other, wiU not be equivalent
for the description of natural phenomena. In
this case we should be constrained to beUeve that
natural laws are capable of being formulated in a
particularly simple manner, and of course only on
condition that, from amongst all possible Galileian
THE PRINCIPLE OF RELATIVITY 17
co-ordinate systems, we should have chosen one
(Ko) of a particular state of motion as our body of
reference. We should then be justified (because
of its merits for the description of natural phe-
nomena) in calling this system "absolutely at
rest," and all other Galileian systems K "in mo-
tion." If, for instance, our embankment were the
system Ko, then our railway carriage would be a
system K, relative to which less simple laws would
hold than with respect to Ko. This duninished
simpUcity would be due to the fact that the carriage
K would be in motion {i.e. "really") with respect
to Ko. In the general laws of nature which have
been formulated with reference to K, the magni-
tude and direction of the velocity of the carriage
would necessarily play a part. We should expect,
for instance, that the note emitted by an organ-
pipe placed with its axis parallel to the direction of
travel would be different from that emitted if the
axis of the pipe were placed perpendicular to this
direction. Now in virtue of its motion in an orbit
round the sun, oiu* earth is comparable with a rail-
way carriage travelling with a velocity of about
30 kilometres per second. If the principle of
relativity were not valid we should therefore expect
that the direction of motion of the earth at any
moment would enter into the laws of nature, and
also that physical systems in their behaviour
would be dependent on the orientation in space
18 SPECIAL THEORY OF RELATIVITY
with respect to the earth. For owing to the altera-
tion in direction of the velocity of revolution of the
earth in the course of a year, the earth cannot be
at rest relative to the hypothetical system ^o
throughout the whole year. However, the most
careful observations have never revealed such
anisotropic properties in terrestrial physical space,
i.e. a physical non-equivalence of different direc-
tions. This is a very powerful argument in favour
of the principle of relativity.
VI
THE THEOREM OF THE ADDITION OF
VELOCITIES EMPLOYED EST CLASSI-
CAL MECHANICS
LET us suppose our old friend the railway
carriage to be travelling along the rails with
a constant velocity v, and that a man
traverses the length of the carriage in the direction
of travel with a velocity w. How quickly, or, in
other words, with what velocity W does the man
advance relative to the embankment during the
process? The only possible answer seems to
result from the following consideration: If the
man were to stand still for a second, he would
advance relative to the embankment through a
distance v equal niunericaUy to the velocity of the
carriage. As a consequence of his walking, how-
ever, he traverses an additional distance w relative
to the carriage, and hence also relative to the
embankment, in this second, the distance w being
numerically equal to the velocity with which he is
walking. Thus in total he covers the distance
W= V +w relative to the embankment in the
second considered. We shall see later that this
result, which expresses the theorem of the addi-
19
«0 SPECIAL THEORY OF RELATIVITY
tion of velocities employed in classical mechamcs,
cannot be maintained; in other words, the law
that we have just written down does not hold in
reality. For the time being, however, we shall
assume its correctness.
vn
THE APPARENT INCOMPATIBILITY OF THE
LAW OF PROPAGATION OF LIGHT WITH
THE PRINCIPLE OF RELATIVITY
THERE ia hardly a simpler law in physics
than that according to which light is propa-
gated in empty space^ Every child at school
knows, or behevesTTe'knows, that this propagation
takes place in straight lines with a velocity
c = 300,000 km. /sec. At all events we know with
great exactness that this velocity is the same for
all colours, because if this were not the case, the
minimum of emission would not be observed
simultaneously for different colours during the
eclipse of a fixed star by its dark neighbour. By
means of similar considerations based on observa-
tions of double stars, the Dutch astronomer De
Sitter was also able to show that the velocity of
propagation of light cannot depend on the velocity
of motion of the body emitting the light. The
assumption that this velocity of propagation is
dependent on the direction "in space" is in itself
improbable.
In short, let us assimie that the simple law of
the constancy of the velocity of light c (in vacuxmi)
21
28 SPECIAL THEORY OF RELATIVITY
is justifiably believed by the duld at school. Who
would imagine that this simple law has pltmged
the conscientiously thoughtful physicist into the
greatest intellectual difficulties? Let us consider
how these difliculties arise.
Of course we must refer the process of the
propagation of hght (and indeed every other
process) to a rigid reference-body (co-ordinate
system). As such a system let us again choose
our embankment. We shall imagine the air above
it to have been removed. If a ray of light be sent
along the embankment, we see from the above
that the tip of the ray wiU be transmitted with
the velocity c relative to the embankment. Now
let us suppose that oiu: railway carriage is again
travelling along the railway lines with the velocity
V, and that its direction is the same as that of the
ray of Kght, but its velocity of course much less.
Let us inquire about the velocity of propagation
of the ray of light relative to the carriage. It is
obvious that we can here apply the consideration
of the previous section, since the ray of Ught plays
the part of the man walking along relatively to
the carriage. The velocity W of the man relative
to the embankment is here replaced by the velocity
of Kght relative to the embankment, w is the
required velocity of Hght with respect to the
carriage, and we have
w = c — p.
THE PROPAGATION OF LIGHT 23
The velocity of propagation of a ray of light
relative to the carriage thus comes out smaller
than c.
But this result comes into conflict with the
principle of relativity set forth ia Section V. For,
like every other general law of nature, the law of
the transmission of light in vacuo must, according
to the principle of relativity, be the same for the
railway carriage as reference-body as when the
rails are the body of reference. But, from our
above consideration, this would appear to be im-
possible. If every ray of Ught is propagated rela-
tive to the embankment with the velocity c, then
for this reason it would appear that another law
of propagation of light must necessarily hold with
respect to the carriage — a result contradictory to
the principle of relativity.
In view of this dilemma there appears to be
nothing else for it than to abandon either the
principle of relativity or the simple law of the
propagation of light in vacuo. Those of you who
have carefully followed the preceding discussion
are almost sure to expect that we shoxild retain
the principle of relativity, which appeals so con-
vincingly to the intellect because it is so natural
and simple. The law of the propagation of light
in vacuo would then have to be replaced by a
more complicated law conformable to the principle
of relativity. The development of theoretical
24 SPECIAL THEORY OF RELATIVITY
physics shows, however, that we cannot pursue
this course. The epoch-making theoretical in-
vestigations of H. A. Lorentz on the electrody-
namical and optical phenomena connected with
moving bodies show that experience in this domain
leads conclusively to a theory of electromagnetic
phenomena, of which the law of the constancy of
the velocity of Ught in vacuo is a necessary conse-
quence. Prominent theoretical physicists were
therefore .more inclined to reject the principle of
relativity, in spite of the fact that no empirical
data had been found which were contradictory to
this principle..
At'thls Juncture the theory of relativity entered
the arena. As a result of an analysis of the physical
conceptions of time and space, it became evident
that in reality there is not the least incompatibility
between the principle of relativity and the law of
propagation of light, and that by systematically
holding fast to both these laws a logically rigid
theory could be arrived at. This theory has been
called the special theory of relativity to distinguish
it from the extended theory, with which we shall
deal later. In the following pages we shall present
the fundamental ideas of the special theory of
relativity.
vm
ON THE roEA OF TIME IN PHYSICS
LIGHTNING has struck the rails on our rail-
way embankment at two places A and B
far distant from each other. I make the
additional assertion that these two Hghtning
flashes occurred simultaneously. If I ask you
whether there is sense in this statement, you
will answer my question with a decided "Yes."
But if I now approach you with the request to
explain to me the sense of the statement more
precisely, you find after some consideration that
the answer to this question is not so easy as it
appears at first sight.
After some time perhaps the following answer
would occur to you: "The significance of the
statement is clear in itself and needs no fiurther
explanation; of course it would require some con-
sideration if I were to be commissioned to deter-
mine by observations whether in the actual case
the two events took place simultaneously or not."
I cannot be satisfied with this answer for the follow-
ing reason. Supposing that as a result of ingenious
considerations an able meteorologist were to dis-
ss
26 SPECIAL THEORY OF RELATIVITY
cover that the lightning must always strike the
places A and B simultaneously, then we should be
faced with the task of testing whether or not this
theoretical result is in accordance with the reality.
We encoimter the same difficulty with aU physical
statements in which the conception "simultane-
ous" plays a part. The concept does not exist
for the physicist until he has the possibility of
discovering whether or not it is fulfilled in an
actual case. We thus require a definition of
simultaneity such that this definition supplies us
with the method by means of which, in the present
case, he can decide by experiment whether or not
both the Ughtning strokes ocoirred simultane-
ously. As long as this requirement is not satisfied,
I allow myself to be deceived as a physicist (and
of course the same appUes if I am not a physicist),
when I imagine that I am able to attach a meaning
to the statement of simultaneity. (I would ask
the reader not to proceed farther until he is fully
convinced on this point.)
After thinking the matter over for some time
you then offer the following suggestion with which
to test simultaneity. By measuring along the
rails, the connecting line AB should be measured
up and an observer placed at the mid-point M
of the distance AB. This observer should be
supplied with an arrangement (e.g. two mirrors
inclined at 90°) which allows him visually to ob-
roEA OF TIME IN PHYSICS «7
serve both places A and B at the same time. If
the observer perceives the two flashes of lightning
at the same time, then they are simultaneous.
I am very pleased with this suggestion, but for
all that I cannot regard the matter as qxiite settled,
because I feel constrained to raise the following
objection: "Your definition would certainly be
right, if I only knew that the light by means of
which the observer at M. perceives the Hghtning
flashes travels along the length A > M with the
same velocity as along the length B > M.
But an examination of this supposition would only
be possible if we already had at our disposal the
means of measuring time. It would thus appear
as though we were moving here in a logical circle."
After further consideration you cast a somewhat
disdainful glance at me — and rightly so — and
you declare: "I maintain my previous definition
nevertheless, because in reahty it assumes ab-
solutely nothing about light. There is only one
demand to be made of the definition of simulta-
neity, namely, that in every real case it must
supply us with an empirical decision as to whether
or not the conception that has to be defined is
fulfilled. That my definition satisfies this demand
is indisputable. That light requires the same
time to traverse the path A — -> M as for the
path B > M is in reality neither a supposition
nor a hypothesis about the physical nature of light,
28 SPECIAL THEORY OF RELATIVITY
but a stipulation which I can make of my own
freewill in order to arrive at a definition of
simultaneity."
It is clear that this definition can be used to
give an exact meaning not only to two events, but
to as many events as we care to choose, au'' in-
dependently of the positions of the scenes of the
events with respect to the body of reference'
(here the railway embankment). We are thus led
also to a definition of "time" in physics. For
this purpose we suppose that clocks of identical
construction are placed at the points A, B and C
of the railway line (co-ordinate system), and that
they are set in such a manner that the positions
of their pointers are simultaneously (in the above
sense) the same. Under these conditions we
imderstand by the "time" of an event the reading
(position of the hands) of that one of these clocks
which is in the immediate vicinity (in space) of
the event. In this manner a time-value is asso-
ciated with every event which is essentially capable
of observation.
This stipulation contains a further physical
1 We suppose further that, when three events A, B and C take
place in different places in such a manner that, if A is simultaneous
with B, and B is simultaneous with C (simultaneous in the sense of
the above definition), then the criterion for the simultaneity of the
pair of events ^, C is also satisfied. This assumption is a physical
hypothesis about the law of propagation of light; it must certainly
be fulfilled if we are to maintain the law of the constancy of the
velocity of light in vacuo.
IDEA OF TIME IN PHYSICS 29
hypothesis, the validity of which will hardly be
doubted without empirical evidence to the con-
trary. It has been assumed that all these clocks
go at the same rate if they are of identical construc-
tion. Stated more exactly: When two clocks
arranged at rest in different places of a reference-
body are set in such a manner that a particular
position of the pointers of the one clock is simtd-
taneous (in the above sense) with the same position
of the pointers of the other clock, then identical
"settings" are always simultaneous (in the sense
of the above definition).
IX
THE RELATIVITY OF SIMULTANEITY
UP to now oixr considerations have been re-
ferred to a particular body of reference,
which we have styled a "railway embank-
ment." We suppose a very long train travelling
along the rails with the constant velocity v and
in the direction indicated in Fig. i. People
travelling in this train will with advantage use
the train as a rigid reference-body (co-ordinate
system); they regard all events in reference to
r M'- — » y > Train
'^ I ! -4 ^
M B EmbajUcment
Fig. I.
the train. Then every event which takes place
along the line also takes place at a particular
point of the train. Also the definition of simul-
taneity can be given relative to the train in exactly
the same way as with respect to the embankment.
As a natural consequence, however, the following
question arises:
Are two events {e.g. the two strokes of lightning
A and B) which are simultaneous with reference to
30
RELATIVITY OP SIMULTANEITY 31
the railway embankment also simultaneous relatively
to the train? We shall show directly that the
answer must be in the negative.
When we say that the lightning strokes A and B
are simultaneous with respect to the embankment,
we mean: the rays of light emitted at the places
A and B, where the Ughtning occurs, meet each
other at the mid-point M of the length A > B
of the embankment. But the events A and B
also correspond to positions A and B on the
traia. Let M' be the mid-point of the distance
A > B on the travelling train. Just when the
flashes ^ of lightning occur, this point M' naturally
coincides with the point M, but it moves towards
the right in the diagram with the velocity v of
the train. If an observer sitting in the position
M' in the train did not possess this velocity, then
he would remain permanently at M, and the light
rays emitted by the flashes of lightning A and B
would reach him simultaneously, i.e. they would
meet just where he is situated. Now in reality
(considered with reference to the railway embank-
ment) he is hastening towards the beam of light
coming from B, whilst he is riding on ahead of the
beam of light coming from A. Hence the observer
will see the beam of light emitted from B earlier
than he will see that emitted from A. Observers
who take the railway train as their reference-body
* As judged from the embankment.
32 SPECIAL THEORY OF RELATIVITY
must therefore come to the conclusion that the
lightning flash B took place earUer than the light-
ning flash A. We thus arrive at the important
result:
Events which are simultaneous with reference
to the embankment are not simultaneous with
respect to the train, and vice versa (relativity of
simultaneity). Every reference-body (co-ordinate
system) has its own particular time; unless we
are told the reference-body to which the statement
of time refers, there is no meaning in a statement
of the time of an event.
Now before the advent of the theory of relativity
it had always tacitly been assumed in physics
that the statement of time had an absolute
significance, i.e. that it is independent of the state
of motion of the body of reference. But we have
just seen that this assumption is incompatible
with the most natural definition of simultaneity;
if we discard this assumption, then the conflict
between the law of the propagation of light in
vacuo and the principle of relativity (developed
in Section VII) disappears.
We were led to that conflict by the considera-
tions of Section VI, which are now no longer
tenable. In that section we concluded that the
man in the carriage, who traverses the distance
w per second relative to the carriage, traverses the
same distance also with respect to the embank-
RELATIVITY OF SIMULTANEITY 33
ment in each second of time. But, according to
the foregoing considerations, the time required by
a particular occurrence with respect to the carriage
must not be considered equal to the duration of
the same occurrence as judged from the embank-
ment (as reference-body). Hence it cannot be
contended that the man in walking travels the
distance w relative to the railway Hne in a time
which is equal to one second as judged from the
embankment.
Moreover, the considerations of Section VI are
based on yet a second assmnption, which, in the
light of a strict consideration, appears to be
arbitrary, although it was always tacitly made
even before the introduction of the theory of
relativity.
ON THE RELATIVITY OF THE CONCEPTION
OF DISTANCE
LET us consider two particular points on the
train* travelling along the embankment
with the velocity v, and inquire as to their
distance apart. We already know that it is neces-
sary to have a body of reference for the measure-
ment of a distance, with respect to which body
the distance can be measured up. It is the simplest
plan to use the train itself as the reference-body
(co-ordinate system). An observer in the train
measures the interval by marking oflE his measur-
ing-rod in a straight line {e.g. along the floor of
the carriage) as many times as is necessary to
take him from the one marked point to the other.
Then the nmnber which tells us how often the
rod has to be laid down is the required distance.
It is a different matter when the distance has
to be judged from the railway line. Here the
following method suggests itself. If we call A'
and B' the two points on the train whose distance
apart is required, then both of these points are
* e.g. the middle of the first and of the hundredth carriage.
Si
THE RELATIVITY OF DISTANCE 35
moving with the velocity v along the embankment.
In the first place we require to determine the
points A and B of the embankment which are
just being passed by the two points A' and B'
at a particular time t — judged from the embank-
ment. These points A and B of the embankment
can be determined by applying the definition of
time given ia Section VIII. The distance between
these points A and B is then measured by repeated
application of the measuring-rod along the em-
bankment.
A priori it is by no means certain that this last
measurement wUl supply us with the same result
as the first. Thus the length of the train as
measured from the embankment may be different
from that obtained by measuring in the train
itself. This circumstance leads us to a second
objection which must be raised against the ap-
parently obvious consideration of Section VI.
Namely, if the man in the carriage covers the
distance w ia a unit of time — measured from the
train, — then this distance — as measured from the
embankment — is not necessarily also equal to w.
XI
THE LORENTZ TRANSFORMATION
THE results of the last three sections show
that the apparent incompatibility of the
law of propagation of light with the principle
of relativity (Section VII) has been derived by
means of a consideration which borrowed two
unjustifiable hypotheses from classical mechanics;
these are as foUows:
(i) The time-interval (time) between two events
is independent of the condition of motion
of the body of reference.
(2) The space-iaterval (distance) between two
points of a rigid body is independent of
the condition of motion of the body of
reference.
If we drop these hypotheses, then the dilemma
of Section VII disappears, because the theorem of
the addition of velocities derived in Section VI
becomes invalid. The possibiHty presents itself
that the law of the propagation of light in vacuo
may be compatible with the principle of relativity,
and the question arises: How have we to modify
the considerations of Section VI in order to remove
36
THE LORENTZ TRANSFORMATION 37
the apparent disagreement between these two
fundamental results of experience? This question
leads to a general one. In the discussion of
Section VI we have to do with places and times
relative both to the train and to the embankment.
How are we to find the place and time of an event
in relation to the train, when we know the place
and time of the event with respect to the railway
embankment? Is there a thinkable answer to this
question of such a nature that the law of transmis-
sion of hght in vacuo does not contradict the
principle of relativity? In other words: Can we
conceive of a relation between place and time of
the individual events relative to both reference-
bodies, such that every ray of Ught possesses the
velocity of transmission c relative to the embank-
ment and relative to the train? This question
leads to a quite definite positive answer, and to a
perfectly definite transformation law for the space-
time magnitudes of an event when changing over
from one body of reference to another.
Before we deal with this, we shall introduce the
following incidental consideration. Up to the
present we have only considered events taking
place along the embankment, which had mathe-
matically to assume the fimction of a straight line.
In the manner indicated in Section II we can
imagine this reference-body supplemented later-
ally and in a vertical direction by means of a
S8 SPECIAL THEORY OF RELATIVITY
framework of rods, so that an event which takes
place anywhere can be localised with reference
to this framework. Similarly, we can imagine
the train travelling with the velocity v to be
continued across the whole of space, so that every
event, no matter how far ofE it may be, could also
be localised with respect to the second framework.
Without committing any fundamental error, we
can disregard the fact that in reality these frame-
works would continually interfere with each other,
owing to the impenetrability of solid bodies. In
every such framework we imagine three surfaces
perpendicular to each other marked out, and
designated as "co-ordinate planes" ("co-ordinate
S3^tem"). A co-ordinate system K then corre-
sponds to the embankment, and a co-ordinate
system K' to the train. An event, wherever it
may have taken place, would be fixed in space
with respect to K by the three perpendiculars
X, y, z on the co-ordinate planes, and with regard
to time by a time-value t. Relative to K', the
same event would be fixed in respect of space and
time by corresponding values x', y', z', /', which
of course are not identical with x, y, z, t. It has
already been set forth in detail how these magni-
tudes are to be regarded as results of physical
measurements.
Obviously our problem can be exactly formu-
lated in the following manner. What are the
THE LORENTZ TRANSFORMATION 39
*:v
values a/, /, z', ^ of an event with respect to K',
when the magnitudes x, y, z, t, of the same event
with respect to K are given? The relations must
be so chosen that the law
of the transmission of
light in vacuo is satisfied
for one and the same ray
of light (and of course for
every ray) with respect to
K and K'. For the rela-
tive orientation in space
of the co-ordinate systems indicated in the diagram
(Fig. 2), this problem is solved by means of the
eqiiations:
, X — vt
Fig. 2.
nT
t
= z
f^
I?
>R
This system of equations is known as the "Lorentz
transformation." ^
K in place of the law of transmission of light we
had taken as our basis the tacit assimiptions of
the older mechanics as to the absolute character
* A simple derivation of the Lorentz transfoimation is given in
Appendir I.
40 SPECIAL THEORY OF RELATIVITY
of times and lengths, then instead of the above we
should have obtained the following equations:
x' =x — vt
y = y
n = t.
This system of equations is often termed the
" Galilei transformation." The Galilei transforma-
tion can be obtauied from the Lorentz trans-
formation by substituting an infinitely large value
for the velocity of Ught c in the latter trans-
formation.
Aided by the following illustration, we can
readily see that, ia accordance with the Lorentz
transformation, the law of the transmission of
hght in vacuo is satisfied both for the reference-
body K and for the reference-body K'. A light-
signal is sent along the positive a>-axis, and this
Ught-stimulus advances in accordance with the
equation
X = ct,
i.e. with the velocity c. According to the equations
of the Lorentz transformation, this simple rela-
tion between x and t involves a relation between
x' and /'. In point of fact, if we substitute for
X the value ct in the first and fourth equations of
the Lorentz transformation, we obtain:
^, (c-v)t
THE LORENTZ TRANSFORMATION 41
]t
from which, by division, the expression
x' = ct'
immediately follows. If referred to the system K',
the propagation of light takes place according to
this equation. We thus see that the velocity of
transmission relative to the reference-body K' is
also equal to c. The same result is obtained for
rays of light advancing in any other direction
whatsoever. Of course this is not surprising,
since the equations of the Lorentz transformation
were derived conformably to this point of view.
xn
THE BEHAVIOUR OF MEASURING-RODS "AND
CLOCKS IN MOTION ' '
I PLACE a metre-rod iB the aj'-axis of K' in
such a manner that one end (the beginning)
coincides with the point x' = o, whilst the
other end (the end of th rod) coincides with the
point x' = I. What is the length of the metre-
rod relatively to the system K? In order to learn
this, we need only ask where the beginning of the
rod and the end of the rod lie with respect to K
at a particvdar time t of the system K. By means
of the first equation of the Lorentz transformation
the values of these two points at the time t =o
can be shown to be
I ^
*(begiiming of rod) " °"\ ^ ^s
I ^
*(endofrod) "^'V^"^'
I «*'
the distance between the points being -J i
But the metre-rod is moving with the velocity v
relative to K. It therefore follows that the length
of a rigid metre-rod moving in the direction of its
length with a velocity » is V i - d*/c* of a metre.
The rigid rod is thus shorter when in motion than
42
RODS AND CLOCKS IN MOTION 4S
when at rest, and the more quickly it is moving,
the shorter is the r od. For the velocity v = c
we should have V i - ^V c* = o, and for still
greater velocities the square-root becomes im-
aginary. From this we conclude that in the
theory of relativity the velocity c plays the part
of a limiting velocity, which can neither be reached
nor exceeded by any real body.
Of course this feature of the velocity c as a
Umiting -velocity also clearly follows from the
equations of the Lorentz transformation, for these
become meaningless if we choose values of v
greater than c.
If, on the contrary, we had considered a metre-
rod at rest in the x-axis with respect to K, then we
should have found that the length of the rod as
judged from K' would have been ^ x-t?- 1 c^; this
is quite in accordance with the principle of rela-
tivity which forms the basis of our considerations.
A priori it is quite clear that we must be able to
learn something about the physical behaviour of
measuring-rods and clocks from the equations of
transformation, for the magnitudes x, y, z, t, are
nothing more nor less than the results of measure-
ments obtainable by means of measuring-rods and
clocks. If we had based our considerations on the
Galilei transformation we should not have ob-
tained a contraction of the rod as a consequence
of its motion.
44 SPECIAL THEORY OF RELATIVITY
Let us now consider a seconds-clock which is
permanently situated at the origin (*' = o) of K'.
if = o and tf = i are two successive ticks of this
clock. The first and fourth equations of the
Lorentz transformation give for these two ticks:
/ = o
and
t =
4^-
As judged from K, the dock is moving with
the velocity v; as judged from this reference-body,
the time which elapses between two strokes of the
clock is not one second, but ■ . seconds, i.e.
a somewhat larger time. As a consequence of its
motion the clock goes more slowly than when at
rest. Here also the velocity c plays the part of
an unattainable limiting velocity.
xm
THEOREM OF THE ADDITION OF VELOCITIES.
THE EXPERIMENT OF FIZEAU
NOW in practice we can move clocks and
measuring-rods only with velocities that
are small compared with the velocity of
light; hence we shall hardly be able to compare
the results of the previous section directly with
the reality. But, on the other hand, these results
must strike you as being very singular, and for
that reason I shall now draw another conclusion
from the theory, one which can easUy be derived
from the foregoing'cdnsiderations,' andwhich has
been most elegantly confirmed by experiments
In Section VI we derived the theorem of the
addition of velocities in one direction in the form
which also results from the hypotheses of classical
mechanics. This theorem can also be deduced
readily from the Galilei transformation (Section
XI). In place of the man walking inside the
carriage, we introduce a point moving relatively
to the co-ordinate system K' in accordance with
the equation
x' = wt'.
By means of the first and fourth equations of the
45
46 SPECIAL THEORY OF RELATIVITY
Galilei transfonnation we can express a;' and i
in terms of x and /, and we then obtain
a; = (z) + 110)1.
This equation expresses nothing else than the law
of motion of the point with reference to the system
K (of the man with reference to the embankment).
We denote this velocity by the symbol W., and we
then obtain, as in Section VI,
W = -o-\-w (A).
But we can carry out this consideration just as
well on the basis of the theory of relativity. In
the equation
x' = wt'
we must then express x' and f in terms of x and t,
making use of the first and fourth equations of the
Lorentz transformation. Instead of the equation
(A) we then obtain the equation
W = '-±^... (B),
^+^
which corresponds to the theorem of addition for
velocities in one direction according to the theory
of relativity. The question now arises as to which
of these two theorems is the better in accord with
experience. On this point we are enHghtened by
a most important experiment which the brilliant
physicist Fizeau performed more than half a
century ago, and which has been repeated since
THE EXPERIMENT OF FIZEAU 47
then by some of the best experimental physicists,
so that there can be no doubt about its result.
TTie experiment is concerned with the following
question. Light travels ia a motionless hquid
with a particular velocity w. How quickly does
it travel in the direction of the arrow in the tube T
(see the accompanying diagram, Fig. 3) when the
liquid above mentioned is flowiug through the
tube with a velocity 11?
In accordance with the principle of relativity
we shall certainly have to take for granted that
the propagation of Hght always takes place with
the same velocity w with respect to the liquid,
whether the latter is in motion with reference to
other bodies or not. The velocity of Hght relative
to the liquid and the velocity of the latter relative
to the tube are thus known, and we require the
velocity of light relative to the tube, s
It is clear that we have the problem of Section
VI again before us. The tube plays the part of
/"
Fig. 3
the railway embankment or of the co-ordinate
system K, the liquid plays the part of the carriage
or of the co-ordinate system K', and finally, the
light plays the part of the man walking along the
carriage, or of the moving pomt in the present
48 SPECIAL THEORY OF RELATIVITY
section. If we denote the velocity of the light
relative to the tube by W, then this is given by
the equation (A) or (B), according as the Galilei
transformation or the Lorentz transformation
corresponds to the facts. Experiment ^ decides in
favour of equation (B) derived from the theory of
relativity, and the agreement is, indeed, very
exact. According to recent and most excellent
measurements by Zeeman, the influence of the
velocity of flow v on the propagation of Hght
is represented by formula (B) to within one
per cent.
Nevertheless we must now draw attention to
the fact that a theory of this phenomenon was
given by H. A. Lorentz long before the statement
of the theory of relativity. This theory was of a
pmrely electrodynamical nature, and was obtained
by the use of particular hypotheses as to the
electromagnetic structure of matter. This circmn-
stance, however, does not in the least diminish
the conclusiveness of the experiment as a crucial
test in favour of the theory of relativity, for the
1 Fizeau found W = w + v{'i ■»), where n= - is the index
of refraction of the liquid. On the other hand, owing to the small-
ness of -J- as compared with i, we can replace (B) in the first place
by W = (w + ») ( I - -3 j, or to the same order of approximation by
ai + 1) ( I — A, which agrees with Fizeau's result.
THE EXPERIMENT OF FIZEAU 49
electrodynamics of Maxwell-Lorentz, on which the
original theory was based, in no way opposes the
theory of relativity. Rather has the latter been
developed from electrodynamics as an astomidingly
simple combination and generahsation of the
hj^otheses, formerly independent of each other,
on which electrodynamics was built.
XIV
THE HEURISTIC VALUE OF THE THEORY OF
RELATIVITY
OUR train of thought in the foregoing pages
can be epitomised in the following manner.
Experience has led to the conviction that,
on the one hand, the principle of relativity holds
true, and that on the other hand the velocity of
transmission of Ught in vacuo has to be considered
equal to a constant c. By uniting these two postu-
lates we obtained the law of transformation for
the rectangular co-ordinates x, y, z and the time
t of the events which constitute the processes of
nature. In this connection we did not obtain
the Galilei transformation, but, differing from
classical mechanics, the Lorentz transformation.
The law of transmission of light, the acceptance
of which is justified by our actual knowledge,
played an important part in this process of thought.
Once in possession of the Lorentz transformation,
however, we can combine this with the principle
of relativity, and simi up the theory thus:
Every general law of nature must be so con-
stituted that it is transformed into a law of
exactly the same form when, instead of the space-
£0
HEURISTIC VALUE OF RELATIVITY 51
time variables x, y, 2, / of the original co-ordinate
system K, we introduce new space-time variables
x', y, 2', t' of a co-ordinate system K'. In this
connection the relation between the ordinary and
the accented magnitudes is given by the Lorentz
transformation. Or, in brief: General laws of
nature are co-variant with respect to Lorentz
transformations.
This is a definite mathematical condition that
the theory of relativity demands of a natural law,
and in virtue of this, the theory becomes a valuable
heuristic aid in the search for general, laws of
nature. If a general law of nature were to be fovmd
which did not satisfy this condition, then at least
one of the two fundamental assumptions of the
theory would have been disproved. Let us now
examine what general results the latter theory
has hitherto evinced.
XV
GENERAL RESULTS OF THE THEORY
IT is clear from our previous considerations that
the (special) theory of relativity has grown
out of electrod3Tiamics and optics. In these
fields it has not appreciably altered the predictions
of theory, but it has considerably simplified the
theoretical structure, i.e. the derivation of laws,
and — what is incomparably more important — it
has considerably reduced the number of inde-
pendent hj^otheses forming the basis of theory.
The special theory of relativity has rendered the
Maxwell-Lorentz theory so plausible, that the
latter would have been generally accepted by
physicists even if experiment had decided less
unequivocally in its favour.
Classical mechanics required to be modified
before it could come into line with the demands
of the special theory of relativity. For the main
part, however, this modification affects only the
laws for rapid motions, in which the velocities of
matter v are not very small as compared with the
velocity of light. We have experience of such
rapid motions only in the case of electrons and
St
GENERAL RESULTS OF THEORY 53
ions; for other motions the variations from the
laws of classical mechanics are too small to make
themselves evident in practice. We shall not
consider the motion of stars mitil we come to
speak of the general theory of relativity. In
accordance with the theory of relativity the
kinetic energy of a material point of mass m is no
longer given by the well-known expression
2
but by the expression
tru?
0>
This expression approaches infinity as the velocity
V approaches the velocity of Ught c. The velocity
must therefore always remain less than c, however
great may be the energies used to produce the
acceleration. If we develop the expression for
the kinetic energy in the form of a series, we
obtain
fnc'+m- + fm'-^+ ....
2 o C
When - is small compared with unity, the third
c
of these terms is always small in comparison with
the second, which last is alone considered in classi-
cal mechanics. The first term mc^ does not contain
the velocity, and requires no consideration if we
54 SPECIAL THEORY OF RELATIVrTY
are only dealing with the question as to how the
energy of a point-mass depends on the velocity.
We shaU speak of its essential significance later.
The most important result of a general character
to which the special theory of relativity has led is
concerned with the conception of mass. Before
the advent of relativity, physics recognised two
conservation laws of fundamental importance,
namely, the law of^the conservation of energy
and the law of the conservation of mass; these
two fimdamental laws appeared to be quite in-
dependent of each other. By means of the
theory of relativity they have been imited into one
law. We shall now briefly consider how this
imification came about, and what meaning is to
be attached to it. |
The principle of relativity'requires that the law
of the conservation of energy should hold not
only with reference to a co-ordinate system K,
but also with respect to every co-ordinate system
K' which is in a state of uniform motion of transla-
tion relative to K, or, briefly, relative to every
"Galileian" system of co-ordinates. In contrast
to classical mechanics, the Lorentz transformation
is the deciding factor in the transition from one
such system to another.
By means of comparatively simple considera-
tions we are led to draw the following conclusion
from these premises, in conjunction with the
GENERAL RESULTS OF THEORY 55
fundamental equations of the electrod3Tiamics of
Maxwell: A body moving with the velocity v,
which absorbs^ an amount of energy £o in the
form of radiation without suffering an alteration
in velocity in the process, has, as a consequence,
its energy increased by an amount
Eo
v/-?
In consideration of the expression given above
for the kinetic energy of the body, the required
energy of the body comes out to be
(r^
v^
V
I - -
c
Thus the body has the same energy as a body
of mass (>» + ^ ) moving with the velocity v.
Hence we can say: If a body takes up an amount
of energy £o, then its inertial mass increases by an
amount -^ ; the inertial mass of a body is not a
constant, but varies according to the change in
the energy of the body. The inertial mass of a
system of bodies can even be regarded as a measure
* £o is the energy taken up, as judged from a co-ordinate system
moving with the body.
56 SPECIAL THEORY OP RELATIVITY
of its energy. The law of the conservation of the
mass of a system becomes identical with the law
of the conservation of energy, and is only valid
provided that the system neither takes up nor
sends out energy. Writing the expression for the
energy ia the form
v/
rf
^-^
we see that the term mc^, which has hitherto
attracted our attention, is nothing else than the
energy possessed by the body ^ before it absorbed
the energy Eg.
A direct comparison of this relation with e^eri-
ment is not possible at the present time, owing to
the fact that the changes in energy Eo to which we
can subject a system are not large enough to make
themselves perceptible as a change in the inertial
-En
mass of the system. -^ is too small in comparison
with the mass m, which was present before the
alteration of the energy. It is owing to this circimi-
stance that classical mechanics was able to es-
tablish successfully the conservation of mass as a
law of iadqpendent vaHdity.
Let me add a final remark of a fundamental
nature. The success of the Faraday-Maxwell
^ As judged from a co-ordinate system moving with the body.
GENERAL RESULTS OF THEORY 57
interpretation of electromagnetic action at a dis-
tance resulted in physicists becoming convinced
that there are no such things as instantaneous
actions at a distance (not involving an inter-
mediary mediiun) of the type of Newton's law of
gravitation. According to the theory of relativity,
action at a distance with the velocity of light
always takes the place of instantaneous action at
a distance or of action at a distance with an in-
finite velocity of transmission. This is connected
with the fact that the velocity c plays a funda-
mental r61e in this theory. In Part II we shaU see
in what way this result becomes modified in the
general theory of relativity.
XVI
EXPERIENCE AND THE SPECIAL THEORY
OF RELATIVITY
TO what extent is the special theory of rela-
tivity supported by experience? This ques-
tion is not easily answered for the reason
already mentioned in connection with the funda-
mental experiment of Fizeau. The special theory
of relativity has crystallised out from the Maxwell-
Lorentz theory of electromagnetic phenomena.
Thus all facts of experience which support the
electromagnetic theory also support the theory of
relativity. As being of particular importance, I
mention here the fact that the theory of relativity
enables us to predict the effects produced on the
light reaching us from the fixed stars. These
results are obtained in an exceedingly simple
manner, and the effects indicated, which are due
to the relative motion of the earth with reference
to those fixed stars, are found to be in accord
with experience. We refer to the yearly move-
ment of the apparent position of the fixed stars
resulting from the motion of the earth round the
sun (aberration), and to the influence of the radial
SB
EXPERIENCE AND RELATIVITY 59
components of the relative motions of the fixed
stars with respect to the earth on the colour of
the light reaching us from them. The latter effect
manifests itself in a slight displacement of the
spectral lines of the Hght transmitted to us from
a fixed star, as compared with the position of the
same spectral lines when they are produced by a
terrestrial source of light (Doppler principle).
The experimental arguments in favour of the
MaxweU-Lorentz theory, which are at the same
time argxmients in favour of the theory of rela-
tivity, are too numerous to be set forth here. In
reahty they limit the theoretical possibilities to
such an extent, that no other theory than that of
Maxwell and Lorentz has been able to hold its
own when tested by experience.
But there are two classes of experimental facts
hitherto obtained which can be represented in the
Maxwell-Lorentz theory only by the introduction
of an aiixUiary hypothesis, which in itself — i.e.
without making use of the theory of relativity —
appears extraneous.
It is known that cathode rays and the so-called
/3-rays emitted by radioactive substances consist
of negatively electrified particles (electrons) of
very small inertia and large velocity. By examin-
ing the deflection of these rays imder the influence
of electric and magnetic fields, we can study the
law of motion of these particles very exactly.
60 SPECIAL THEORY OF RELATIVITY
In the theoretical treatment of these electrons,
we are faced with the diflSculty that electro-
dynamic theory of itself is imable to give an ac-
coimt of their nature. For since electrical masses
of one sign repel each other, the negative electrical
masses constituting the electron would necessarily
be scattered under the influence of their mutual
repulsions, unless there are forces of another kind
operating between them, the nature of which has
hitherto remained obscure to us.^ If we now
assimie that the relative distances between the
electrical masses constituting the electron remain
imchanged during the motion of the electron
(rigid connection in the sense of classical me-
chanics), we arrive at a law of motion of the
electron which does not agree with experience.
Guided by piurely formal points of view, H. A.
Lorentz was the first to introduce the hypothesis
that the particles constituting the electron ex-
perience a contraction in the direction of motion
in consequence of that motion, the amount of this
contraction being proportional to the expression
•*/i = -j- This hypothesis, which is not justifiable
by any electrodynamical facts, supplies us then
with that particidar law of motion which has been
confijmed with great precision in recent years.
^ The general theory of relativity renders it likely that the elec-
trical masses of an electron are held together by gravitational forces.
EXPERIENCE AND RELATIVITY 61
The theory of relativity leads to the same law
of motion, without requiring any special hypothe-
sis whatsoever as to the structure and the be-
haviour of the electron. We arrived at a similar
conclusion in Section XIII in connection with the
experiment of Fizeau, the residt of which is fore-
told by the theory of relativity without the ne-
cessity of drawing on hypotheses as to the physical
nature of the liquid.
The second class of facts to which we have
alluded has reference to the question whether or
not the motion of the earth in space can be made
perceptible in terrestrial experiments. We have
already remarked in Section V that all attempts
of this nature led to a negative result. Before
the theory of relativity was put forward, it was
difficult to become reconciled to this negative
result, for reasons now to be discussed. The in-
herited prejudices about time and space did not
allow any doubt to arise as to the prime importance
of the Galilei transformation for changing over
from one body of reference to another. Now
assuming that the Maxwell-Lorentz equations
hold for a reference-body K, we then find that
they do not hold for a reference-body K' moving
imiformly with respect to K, if we assume that
the relations of the Galileian transformation
exist between the co-ordinates of K and K'. It
thus appears that of aU Galileian co-ordinate
62 SPECIAL THEORY OF RELATIVITY
systems one (K) corresponding to a particular
state of motion is physically imique. This result
was interpreted physically by regarding X" as at
rest with respect to a hypothetical aether of space.
On the other hand, all co-ordinate systems K'
moving relatively to K were to be regarded as in
motion with respect to the aether. To this motion
of K' against the aether ("aether-drift" relative to
K') were assigned the more complicated laws
which were supposed to hold relative to K'.
Strictly speaking, such an aether-drift ought also
to be assumed relative to the earth, and for a
long time the efforts of physicists were devoted
to attempts to detect the existence of an aether-
drift at the earth's surface.
In one of iJie most notable of these attempts
Michelson devised a method which appears as
though it must be decisive. Imagine two mirrors
so arranged on a rigid body that the reflecting
surfaces face each other. A ray of light requires
a perfectly definite time T to pass from one mirror
to the other and back again, if the whole system
be at rest with respect to the aether. It is foimd
by calculation, however, that a slightly different
time r' is required for this process, if the body,
together with the mirrors, be moving relatively
to the aether. Andyet another point: it is shown
by calculation that for a given velocity v with
reference to the aether, this time T is different
EXPERIENCE AND RELATIVITY 63
when the body is moving perpendicularly to the
planes of the mirrors from that resulting when
the motion is parallel to these planes. Although
the estimated difference between these two times
is exceedingly small, Michelson and Morley
performed an experiment involving interference
in which this difference shoiild have been clearly
detectable. But the experiment gave a negative
result — a fact very perplexing to physicists.
Lorentz and FitzGerald rescued the theory from
this difi&culty by assuming that the motion of
the body relative to the aether produces a contrac-
tion of the body in the direction of motion, the
amount of contraction being just sufl&cient to
compensate for the difference in time mentioned
above. Comparison with the discussion in Section
XII shows that also from the standpoint of the
theory of relativity this solution of the difficulty
was the right one. But on the basis of the theory
of relativity the method of interpretation is in-
comparably more satisfactory. According to this
theory there is no such thing as a "specially
favoured" (imique) co-ordinate system to occasion
the introduction of the aether-idea, and hence
there can be no aether-drift, nor any experiment
with which to demonstrate it. Here the contrac-
tion of moving bodies follows from the two hm-
damental principles of the theory without the
introduction of particular hypotheses; and as the
64 SPECIAL THEORY OF REIATIVITY
prime factor involved in this contraction we find,
not the motion in itself, to which we cannot
attach any meaning, but the motion with respect
to the body of reference chosen in the particular
case in point. Thus for a co-ordinate system
moving with the earth the mirror system of
Michelson and Morley is not shortened, but it is
shortened for a co-ordinate system which is at
rest relatively to the sim.
xvn
MINKOWSKI'S FOUR-DIMENSIONAL SPACE
THE non-mathematician is seized by a mys-
terious sKuddering when he hears of "four-
dimensional" things, by a feeUng not unlike
that awakened by thoughts of the occult. And
yet there is no more common-place statement than
that the world in which we Uve is a four-dimen-
sional space-time continuum.
Space is a three-dimensional continuum. By
this we mean that it is possible to 'describe the
position of a point (at rest) by means of three
numbers (co-ordinates) x, y, z, and that there is
an indefinite number of points in the neighbour-
hood of this one, the position of which can be
described by co-ordinates such as Xi, yi, Zi, which
may be as near as we choose to the respective
values of the co-ordinates x, y, z of the first point.
In virtue of the latter property we speak of a
"continuum," and owing to the fact that there
are three co-ordinates we speak of it as being
" three-dimensional."
' Similarly, the world of physical phenomena
which was briefly called "world" by Minkowski
65
66 SPECIAL THEORY OF RELATIVITY
is naturally four-dimensional in the space-time
sense. Fdr it is composed of individual events,
each of which is described by four numbers,
namely, three space co-ordinates x, y, z and a
time co-ordinate, the time-value /. The "world"
is in this sense also a continuum; for to every
event there are as many "neighbouring" events
(realised or at least thinkable) as we care to
choose, the co-ordinates Xi, ji, Zi, h of which differ
by an indefinitely small amount from those of the
event x, y, z, t originally considered. That we
have not been accustomed to regard the world
in this sense as a four-dimensional continuum is
due to the fact that in physics, before the advent
of the theory of relativity, time played a different
and more independent role, as compared with
the space co-ordinates. It is for this reason that
we have been in the habit of treating time as an
independent continuum. As a matter of fact,
according to classical mechanics, time is absolute,
i.e. it is independent of the position and the condi-
tion of motion of the system of co-ordinates. We
see this expressed in the last equation of the
GaUleian transformation {t' = t).
The four-dimensional mode of consideration of
the "world" is natural on the theory of relativity,
since according to this theory time is robbed of its
independence. This is shown by the fourth equa-
tion of the Lorentz transformation:
POUR-DIMENSIONAL SPACE 6T
a ~
\/'-$
Moreover, according to this equation the time
difference At' of two events with respect to K'
does not in general vanish, even when the time
difference At of the same events with reference to
K vanishes. Pure "space-distance" of two events
with respect to K results in "time-distance" of
the same events with respect to K'. But the
discovery of Minkowski, which was of importance
for the formal development of the theory of rela-
tivity, does not He here. It is to be found rather
in the fact of his recognition that the four-dimen-
sional space-time continuum of the theory of rela-
tivity, in its most essential formal properties,
shows a pronounced relationship to the three-
dimensional continuum of EucHdean geometrical
space.* In order to give due prominence to this
relationship, however, we must replace the usual
time co-ordinate t by an imaginary magnitude
V— I. ct proportional to it. Under these condi-
tions, the natural laws satisfying the demands of
the (special) theory of relativity assimie mathe-
matical forms, in which the tune co-ordinate plays
exactly the same r61e as the three space co-
ordinates. Formally, these four co-ordinates
' Cf. the somewhat more detailed discussion in Appendix n.
68 SPECIAL THEORY OF RELATIVITY
correspond exactly to the three space co-ordinates
in Euclidean geometry. It must be clear even to
the non-mathematician that, as a consequence of
this purely formal addition to our knowledge, the
theory perforce gained clearness in no mean
measure.
These inadequate remarks can give the reader
only a vague notion of the important idea con-
tributed by Minkowski. Without it the general
theory of relativity, of which the fundamental ideas
are developed in the following pages, would perhaps
have got no farther than its long clothes. Min-
kowski's work is doubtless difficult of access to
anyone inexperienced in mathematics, but since
it is not necessary to have a very exact grasp of
this work in order to xmderstand the fundamental
ideas of either the special or the general theory of
relativity, I shaU at present leave it here, and
shall revert to it only towards the end of Part II.
PART II
THE GENERAL THEORY OF RELATIVITY
xvm
SPECIAL AND GENERAL PRINCIPLE OF
RELATIVITY
THE basal principle, which was the pivot of all
our previous considerations, was the special
principle of relativity, i.e. the principle of
the physical relativity of all uniform motion. Let
us once more analyse its meaning carefully.
It was at all times clear that, from the point of
view of the idea it conveys to us, every motion
must only be considered as a relative motion.
Returning to the illustration we have frequently
used of the embankment and the railway carriage,
we can express the fact of the motion here taking
place in the foUowiug two forms, both of which
are equally justifiable:
(a) The carriage is in motion relative to the
embankment.
(J) The embankment is in motion relative to
the carriage.
In (a) the embankment, in (6) the carriage,
serves as the body of reference in our statement
60
TO GENERAL THEORY OF RELATIVITT
of the motion taking place. If it is simply a
question of detecting or of describing the motion
involved, it is in principle immaterial to what
reference-body we refer the motion. As already
mentioned, this is self-evident, but it must not be
confused with the much more comprehensive state-
ment called "the principle of relativity," which we
have taken as the basis of our investigations.
The principle we have made use of not only
maintains that we may equally ■«?eU choose the
carriage or the embankment as our reference-body
for the description of any event (for this, too, is
self-evident). Our principle rather asserts what
follows: If we formulate the general laws of
nature as they are obtained from experience, by
making use of
(o) the embankment as reference-body,
(b) the railway carriage as reference-body,
then these general laws of nature (e.g. the laws of
mechanics or the law of the propagation of light
in vacuo) have exactly the same form in both cases.
This can also be expressed as follows: For the
physical description of naturaLprocesses^ nd^gi:.
6T^ the "reference-bodies K, K' is unique— (lit —
"specially marked out") as compared with the
other. Unlike the first, this latter statement need
not of necessity hold a priori; it is not contained
in the conceptions of "motion" and "reference-
SPECIAL AND GENERAL PRINCIPLE 71
body" and derivable from them; only experience
can decide as to its correctness or incorrectness.
Up to the present, however, we have by no
means maintained the equivalence of all bodies
of reference K in connection with the formulation
of natural laws. Our course was more on the
following lines. In the first place, we started out
from the assumption that there exists a reference-
body K, whose condition of motion is such that
the GaUleian law holds with respect to it: A
particle left to itself and sufl&ciently far removed
from all other particles moves uniformly in a
straight line. With reference to K (Galileian
reference-body) the laws of nature were to be as
simple as possible. But in addition to K, all
bodies of reference K' should be given preference
in this sense, and they should be exactly equiva-
lent to K for the formulation of natural laws,
provided that they are in a state of uniform
rectilinear and nonrotary motion with respect to K;
all these bodies of reference are to be regarded
as Galileian reference-bodies. The vaUdity of
the principle of relativity was assumed only for
these reference-bodies, but not for others {e.g.
those possessing motion of a different kind). In
this sense we speak of the special principle of
relativity, or special theory of relativity.
In contrast to this we wish to imderstand by
the "general principle of relativity" the following
72 GENERAL THEORY OF RELATIVITY
statement: All bodies of reference K, K\ etc.,
are equivalent for the description of natural
phenomena (formulation of the general laws of
nature), whatever may be their state of motion.
But before proceeding farther, it ought to be
pointed out that this formulation must be re-
placed later by a more abstract one, for reasons
which wiU become evident at a later stage.
Siace the introduction of the special principle
of relatiAdty has been justified, every intellect
which strives after generalisation must feel the
temptation to venture the step towards the general
principle of relativity. But a simple and ap-
parently quite reliable consideration seems to
suggest that, for the present at any rate, there is
Uttle hope of success iu such an attempt. Let
us imagine ourselves transferred to our old friend
the railway carriage, which is travelling at a
uniform rate. As long as it is moving imiformly,
the occupant of the carriage is not sensible of its
motion, and it is for this reason that he can with-
out reluctance interpret the facts of the case as
indicating that the carriage is at rest, but the
embankment m motion. Moreover, according
to the special principle of relativity, this inter-
pretation is quite justified also from a physical
point of view.
If the motion of the carriage is now changed
into a non-uniform motion, as for instance by a
SPECIAL AND GENERAL PRINCIPLE 73
powerful application of the brakes, then the oc-
cupant of the carriage experiences a correspond-
ingly powerful jerk forwards. The retarded mo-
tion is manifested in the mechanical behaviour
of bodies relative to the person in the railway
carriage. The mechanical behaviour is different
from that of the case previously considered, and
for this reason it would appear to be impossible
that the same mechanical laws hold relatively to
the non-imiformly moving carriage, as hold with
reference to the carriage when at rest or in uni-
form motion. At all events it is clear that the
GaMleian law does not hold with respect to the
non-imiformly moving carriage. Because of this,
we feel compelled at the present jimcture to grant
a kind of absolute physical reality to non-uniform
motion, in opposition to the general principle of
relativity. But in what follows we shall soon
see that this conclusion cannot be maintained.
xrx
THE GRAVITATIONAL FIELD
"TF we pick up a stone and then let it go, why
J^ does it faJl to the ground?" The usual
answer to this question is: "Because it is
attracted by the earth." Modem physics formu-
lates the answer rather differently for the follow-
ing reason. As a result of the more careful study
of electromagnetic phenomena, we have come to
regard action at a distance as a process impossible
without the intervention of some intermediary
medium. If, for instance, a magnet attracts a
piece of iron, we cannot be content to regard this
as meaning that the magnet acts directly on the
iron through the intermediate empty space, but
we are constrained to imagine — after the manner
of Faraday — that the magnet always calls
into being something physically real in the space
aroimd it, that something being what we call a
"magnetic field." In its turn this magnetic field
operates on the piece of iron, so that the latter
strives to move towards the magnet. We shall
not discuss here the justification for this incidental
conception, which is indeed a somewhat arbi-
74
THE GRAVITATIONAL FIELD 75
trary one. We shall only mention that with its
aid electromagnetic phenomena can be theoret-
ically represented much more satisfactorily than
without it, and this appHes particularly to the
transmission of electromagnetic waves. The
effects of gravitation also are regarded in an
analogous manner.
The action of the earth on the stone takes
place indirectly. The earth produces in its sur-
roundings a gravitational field, which acts on the
stone and produces its motion of fall. As we
know from experience, the intensity of the action
on a body diminishes according to a quite definite
law, as we proceed farther and farther away from
the earth. From our point of view this means:
The law governing the properties of the gravita-
tional field in space must be a perfectly definite
one, in order correctly to represent the diminution
of gravitational action with the distance from
operative bodies. It is something like this: The
body (e.g. the earth) produces a field in its imme-
diate neighbourhood directly; the intensity and
direction of the field at points farther removed
from the body are thence determined by the law
which governs the properties in space of the
gravitational fields themselves.
In contrast to electric and magnetic fields, the
gravitational field exhibits a most remarkable
property, which is of fimdamental importance
76 GENERAL THEORY OF RELATIVITY
for what follows. Bodies which are moving under
the sole influence of a gravitational field receive
an acceleration, which does not in the least depend
either on the material or on the physical state of the
body. For instance, a piece of lead and a piece
of wood fall in exactly the same manner ia a
gravitational field {in vacuo), when they start off
from rest or with the same initial velocity. This
law, which holds most accurately, can be expressed
in a different form in the Hght of the following
consideration.
According to Newton's law of motion, we have
(Force) = (inertial mass) X (acceleration),
where the "mertial mass" is a characteristic
constant of the accelerated body. If now gravi-
tation is the cause of the acceleration, we then
have
(Force) = (gravitational mass) X (intensity of the
gravitational field),
where the "gravitational mass" is likewise a
characteristic constant for the body. From these
two relations follows:
<««^»«'«) - 'T^sriT' X <-'-"' "' '^
gravitational field).
If now, as we find from experience, the accelera-
tion is to be independent of the nature and the
condition of the body and always the same for a
THE GRAVITATIONAL FIELD 77
given gravitational field, then the ratio of the
gravitational to the inertial mass must likewise
be the same for all bodies. By a suitable choice
of units we can thus make this ratio equal to
unity. We then have the following law: The
gravitational mass of a body is equal to its inertial
mass.
It is true that this important law had hitherto
been recorded in mechanics, but it had not been
interpreted. A satisfactory interpretation can be
obtained only if we recognise the following fact:
The same quality of a body manifests itself ac-
cording to circumstances as "inertia" or as
"weight" (lit. "heaviness"). In the following
section we shall show to what extent this is
actually the case, and how this question is con-
nected with the general postiilate of relativity.
XX
THE EQUALITY OF INERTIAL AND GRAVITA-
TIONAL MASS AS AN ARGUMENT FOR THE
GENERAL POSTULATE OF RELATIVITY
WE imagine a large portion of empty space,
so far removed from stars and other
appreciable masses that we have before
us approximately the conditions required by the
fundamental law of Galilei. It is then possible
to choose a Galileian reference-body for this part
of space (world), relative to which points at rest
remain at rest and points in motion continue
permanently in imiform rectilinear motion. As
reference-body let us imagine a spacious chest
resembling a room with an observer inside who
is equipped with apparatus. Gravitation nat-
urally does not exist for this observer. He must
fasten himself with strings to the floor, otherwise
the sHghtest impact against the floor will cause
him to rise slowly towards the ceiling of the
room.
To the middle of the lid of the chest is fixed
externally a hook with rope attached, and now a
"being" (what kind of a being is immaterial to
78
INERTIAL AND GRAVITATIONAL MASS T9
us) begins pulling at this with a constant force.
The chest together with the observer then b^in
to move "upwards" with a urdfomaly accelerated
motion. In course of time their velocity will
reach unheard-of values — provided that we are
viewing all this from another reference-body
which is not being pulled with a rope.
But how does the man in the chest regard the
process? The acceleration of the chest will be
transmitted to him by the reaction of the floor
of the chest. He must therefore take up this
pressure by means of his legs i£ he does not wish
to be laid out fuU length on the floor. He is then
standing in the chest in exactly the same way as
anyone stands in a room of a house on our earth.
If he release a body which he previously had in
his hand, the acceleration of the chest wUl no
longer be transmitted to this body, and for this
reason the body will approach the floor of the
chest with an accelerated relative motion. The
observer wiU further convince himself that the
acceleration of the body towards the floor of the chest
is always of the same magnittide, whatever kind of
body he may happen to use for the experiment.
Rel3dng on his knowledge of the gravitational
field (as it was discussed in the preceding section),
the man in the chest will thus come to the con-
clusion that he and the chest are in a gravitational
field which is constant with regard to time. Of
80 GENERAL THEORY OF RELATIVITY
course he will be puzzled for a moment as to why
the chest does not fall m this gravitational field.
Just then, however, he discovers the hook in the
middle of the Ud of the chest and the rope which
is attached to it, and he consequently comes to
the conclusion that the chest is suspended at rest
in the gravitational field.
Ought we to smUe at the man and say that he
errs in his conclusion? I do not beheve we ought
to if we wish to remain consistent; we must rather
admit that his mode of grasping the situation
violates neither reason nor known mechanical
laws. Even though it is being accelerated with
respect to the "Galileian space" first considered,
we can nevertheless regard the chest as being at
rest. We have thus good grounds for extending
the principle of rdativity to include bodies of
reference which are accelerated with respect to
each other, and as a result we have gained a
powerful argimaent for a generahsed postulate
of relativity.
We must note carefully that the possiblility of
this mode of interpretation rests on the fundamen-
tal property of the gravitational field of giving
all bodies the same acceleration, or, what comes
to the same thing, on the law of the equaUty of
inertial and gravitational mass. If this natural
law did not exist, the man in the accelerated chest
would not be able to interpret the behaviour of
INERTIAL AND GRAVITATIONAL MASS 81
the bodies around him on the supposition of a
gravitational field, and he would not be justified
on the grounds of experience in supposing his
reference-body to be "at rest."
Suppose that the man in the chest fixes a rope
to the inner side of the Hd, and that he attaches
a body to the free end of the rope. The result of
this wiU be to stretch the rope so that it will
hang "vertically" downwards. If we ask for an
opinion of the cause of tension in the rope, the
man in the chest will say: "The suspended body
experiences a downward force in the gravitational
field, and this is neutrahsed by the tension of the
rope; what determines the magnitude of the ten-
sion of the rope is the gravitational mass of the
suspended body." On the other hand, an ob-
server who is poised freely in space will interpret
the condition of things thus: "The rope must
perforce take part in the accelerated motion of
the chest, and it transmits this motion to the body
attached to it. The tension of the rope is just
large enough to effect the acceleration of the body.
That which determines the magnitude of the
tension of the rope is the inertial mass of the
body." Guided by this example, we see that our
extension of the principle of relativity implies
the necessity of the law of the equality of inertial
and gravitational mass. Thus we have obtained
a physical interpretation of this law.
82 GENERAL THEORY OP RELATIVITY
From our consideration of the accelerated chest
we see that a general theory of relativity must
yield important riesults on the laws of gravitation.
In point of fact, the systematic piu-suit of the
general idea of relativity has supplied the laws
satisfied by the gravitational field. Before pro-
ceeding farther, however, I must warn the reader
against a misconception suggested by tijese con-
siderations. A gravitational field exists for the
man in the chest, despite the fact that there was
no such field for the co-ordinate system first
chosen. Now we might easily suppose that the
existence of a gravitational field is always only
an apparent one. We might also think that,
regardless of the kind of gravitational field which
may be present, we could always choose another
reference-body such that no gravitational field
exists with reference to it. This is by no means
true for aU gravitational fields, but only for those
of quite special form. It is, for mstance, im-
possible to choose a body of reference such that,
as judged from it, the gravitational field of the
earth (in its entirety) vanishes.
We can now appreciate why that argument is
not convincing, which we brought forward against
the general principle of relativity at the end of
Section XVIII. It is certainly true that the
observer in the railway carriage experiences a
jerk forwards as a result of the application of the
INERTIAL AND GRAVITATIONAL MASS 83
brake, and that he recognises in this the non-
umformity of motion (retardation) of the carriage.
But he is compelled by nobody to refer this jerk
to a "real" acceleration (retardation) of the
carriage. He might also interpret his experience
thus: "My body of reference (the carriage)
remains permanently at rest. With reference to
it, however, there exists (during the period of
application of the brakes) a gravitational field
which is directed forwards and which is variable
with respect to time. Under the influence of this
field, the embankment together with the earth
moves non-uniformly in such a manner that their
original velocity in the backwards direction is
continuously reduced."
XXI
IN WHAT RESPECTS ARE THE FOUNDATIONS
OF CLASSICAL MECHANICS AND OF THE
SPECIAL THEORY OF RELATIVITY UN-
SATISFACTORY?
WE have already stated several times that
classical mechanics starts out from the
following law: Material particles suf-
ficiently far removed from other material particles
continue to move uniformly in a straight line
or continue in a state of rest. We have also
repeatedly emphasised that this fundamental law
can only be vahd for bodies of reference K which
possess certain unique states of motion, and which
are in imiform translational motion relative to
each other. Relative to other reference-bodies
K the law is not valid. Both in classical mechanics
and in the special theory of relativity we there-
fore differentiate between reference-bodies K
relative to which the recognised "laws of nature"
can be said to hold, and reference-bodies K
relative to which these laws do not hold.
But no person whose mode of thought is logical
can rest satisfied with this condition of things.
He asks: "How does it come that certain refer-
84
MECHANICS AND RELATIVITY 85
ence-bodies (or their states of motion) are given
priority over other reference-bodies (or their
states of motion)? What is the reason for this
preference? In order to show clearly what I mean
by this question, I shall make use of a comparison.
I am standing in front of a gas range. Stand-
ing alongside of each other on the range are two
pans so much alike that one may be mistaken for
the other. Both are half full of water. I notice
that steam is being emitted continuously from the
one pan, but not from the other. I am surprised at
this, even if I have never seen either a gas range
or a pan before. But if I now, notice a luminous
something of bluish colour under the first pan but
not under the other, I cease to be astonished, even
if I have never before seen a gas flame. For I
can only say that this bluish something will cause
the emission of the steam, or at least possibly it
may do so. If, however, I notice the bluish
something in neither case, and if I observe that
the one continuously emits steam whilst the
other does not, then I shall remain astonished
and dissatisfied until I have discovered some
circumstance to which I can attribute the different
behaviour of the two pans.
Analogously, I seek in vain for a real something
in classical mechanics (or in the special theory
of relativity) to which I can attribute the different
behaviour of bodies considered with respect to
86 GENERAL THEORY OF RELATIVITY
the reference-systems K and K'.'^ Newton saw
this objection and attempted to invalidate it, but
without success. But E. Mach recognised it
most clearly of all, and because of this objection
he claimed that mechanics must be placed on a
new basis. It can only be got rid of by means of
a physics which is conformable to the general
principle of relativity, since the equations of such
a theory hold for every body of reference, whatever
may be its state of motion.
1 The objection is of importance more especially when the state
of motion of the reference-body is of such a nature that it does not
require any external agency for its maintenance, e.g. in the case when
the reference-body is rotating uniformly.
XXII
A FEW INFERENCES FROM THE GENERAL
PRINCIPLE OF RELATIVITY
THE considerations of Section XX show that
the general principle of relativity puts
us in a position to derive properties of the
gravitational field in a purely theoretical manner.
Let us suppose, for instance, that we know the
space-time "course" for any natural process
whatsoever, as regards the manner in which it
takes place in the GaHleian domain relative to a
GaUleian body of reference K. By means of
purely theoretical operations {i.e. simply by cal-
culation) we are then able to find how this known
natural process appears, as seen from a reference-
body K' which is accelerated relatively to K.
But since a gravitational field exists with respect
to this new body of reference K', our consideration
also teaches us how the gravitational field in-
fluences the process studied.
For example, we learn that a body which is
in a state of uniform rectilinear motion with
respect to K (in accordance with the law of
Galilei) is executing an accelerated and in general
8r
88 GENERAL THEOEY OF RELATIVITY
curvilinear motion with respect to the accelerated
reference-body K' (chest). This acceleration or
curvature corresponds to the influence on the
moving body of the gravitational field prevailing
relatively to K' . It is known that a gravita-
tional field influences the movement of bodies in
this way, so that our consideration supplies us
with nothing essentially new.
However, we obtain a new result of fundamental
importance when we carry out the analogous
consideration for a ray of light. With respect
to the Galileian reference-body K, such a ray of
Hght is transmitted rectilinearly with the velocity
c. It can easily be shown that the path of the
same ray of light is no longer a straight line when
we consider it with reference to the accelerated
chest (reference-body i?'). From this we con-
clude, that, in general, rays of light are propagated
curvilinearly in gravitational fields. In two re-
spects this result is of great importance.
In the first place, it can be compared with the
reality. Although a detailed examination of the
question shows that the curvature of light rays
required by the general theory of relativity is
only exceedingly small for the gravitational fields
at our disposal in practice, its estimated magni-
tude for light rays passing the sun at grazing
incidence is nevertheless 1-7 seconds of arc. This
ought to manifest itself in the following way.
INFERENCES FROM RELATIVITY 89
As seen from the earth, certain fixed stars appear
to be in the neighbourhood of the sun, and are
thus capable of observation during a total eclipse
of the sun. At such times, these stars ought to
appear to be displaced outwards from the sun
by an amount indicated above, as compared with
their apparent position in the sky when the sun
is situated at another part of the heavens. The
examination of the correctness or otherwise of
this deduction is a problem of the greatest im-
portance, the early solution of which is to be
expected of astronomers.^
In the second place our result shows that, ac-
cording to the general theory of relativity, the
law of the constancy of the velocity of hght in
vacuo, which constitutes one of the two funda-
mental assiunptions in the special theory of
relativity and to which we have already frequently
referred, cannot claim any unlimited validity.
A curvature of rays of light can only take place
when the velocity of propagation of light varies
with position. Now we might think that as a
consequence of this, the special theory of relativity
and with it the whole theory of relativity would
be laid in the dust. But in reality this is not the
' By means of the star photographs of two expeditions equipped
by a Joint Committee of the Royal and Royal Astronomical Societies,
the existence of the deflection of light demanded by theory was con-
firmed during the solar eclipse of 29th May, 1919. (Cf. Appendix
III.)
90 GENERAL THEORY OF RELATIVITY
case. We can only conclude that the special
theory of relativity cannot claim an unliinited
domaui of validity; its resiilts hold only so long
as we are able to disregard the influences of
gravitational fields on the phenomena {e.g. of
light).
Since it has often been contended by oppo-
nents of the theory of relativity that the special
theory of relativity is overthrown by the general
theory of relativity, it is perhaps advisable to make
the facts of the case clearer by means of an
appropriate comparison. Before the development
of electrodynamics the laws of electrostatics
were looked upon as the laws of electricity.
At the present time we know that electric
fields can be derived correctly from elec-
trostatic considerations only for the case, which
is never strictly realised, in which the electrical
masses are quite at rest relatively to each other,
and to the co-ordinate system. Should we be
justified in saying that for this reason electro-
statics is overthrown by the field-equations of
Maxwell in electrodynamics? Not in the least.
Electrostatics is contained in electrodynamics
as a limiting case; the laws of the latter lead
directly to those of the former for the case in which
the fields are invariable with regard to time.
No fairer destiny covdd be allotted to any physical
theory, than that it should of itself point out the
INFERENCES FROM RELATIVITY 91
way to the introduction of a more comprehensive
theory, in which it lives on as a limiting case.
In the example of the transmission of light just
dealt with, we have seen that the general theory
of relativity enables us to derive theoretically
the influence of a gravitational field on the course
of natural processes, the laws of which are already
known when a gravitational field is absent. But
the most attractive problem, to the solution of
which the general theory of relativity supplies
the key, concerns the investigation of the laws
satisfied by the gravitational field itself. Let us
consider this for a moment.
We are acquainted with space-time domains
which behave (approximately) in a "Galileian"
fashion imder suitable choice of reference-body,
i.e. domains in which gravitational fields are
absent. If we now refer such a domain to a
reference-body K' possessing any kind of motion,
then relative to K' there exists a gravitational
field which is variable with respect to space and
time.* The character of this field will of course
depend on the motion chosen for K'. Accord-
ing to the general theory of relativity, the general
law of the gravitational field must be satisfied
for all gravitational fields obtainable in this way.
Even though by no means all gravitational fields
' This follows from a generalisation of the discussion in Sec-
tion XX.
92 GENERAL THEORY OF RELATIVITY
can be produced in this way, yet we may enter-
tain the hope that the general law of gravitation
will be derivable from such gravitational fields of
a special kind. This hope has been realised~itt-
the most beautiful _ maimer,^ But betwee n the
clear vision of this goal and its actual realisation
it was necessary to surmoimt a serious difi&culty,
and as this lies deep at the root of things, I dare
not withhold it from the reader. We require
to extend our ideas of the space-time continuum
still farther.
xxm
BEHAVIOUR OF CLOCKS AND MEASURING-
RODS ON A ROTATING BODY
OF REFERENCE
HITHERTO I have purposely refrained
from speaking about the physical in-
terpretation of space- and time-data in
the case of the general theory of relativity. As a
consequence, I am guilty of a certain slovenliness
of treatment, which, as we know from the special
theory of relativity, is far from beiug unim-
portant and pardonable. It is now high time
that we remedy this defect; but I would mention
at the outset, that this matter lays no smaU claims
on the patience and on the power of abstraction
of the reader.
We start off again from quite special cases,
which we have frequently used before. Let us
consider a space-time domain in which no gravi-
tational field exists relative to a reference-body
K whose state of motion has been suitably chosen.
K is then a Galileian reference-body as regards
the domain considered, and the results of the
special theory of relativity hold relative to K.
Let us suppose the same domain referred to a
93
94 GENERAL THEORY OP RELATIVITY
second body of reference K', which is rotating
uniformly with respect to K. In order to fix our
ideas, we shall imagine K' to be in the form of a
plane circular disc, which rotates uniformly in
its own plane about its centre. An observer
who is sitting eccentrically on the disc K' is
sensible of a force which acts outwards in a radial
direction, and which would be interpreted as an
effect of inertia (centrifugal force) by an observer
<vho was at rest with respect to the original
reference-body K. But the observer on the disc
may regard his disc as a reference-body which
is "at rest"; on the basis of the general principle
of relativity he is justified in doing this. The
force acting on himself, and in fact on aU other
bodies which are at rest relative to the disc, he
regards as the effect of a gravitational field.
Nevertheless, the space-distribution of this gravi-
tational field is of a kind that would not be possible
on Newton's theory of gravitation.* But since
the observer beUeves in the general theory of
relativity, this does not disturb him; he is quite
in the right when he beUeves that a general law
of gravitation can be formulated — a law which
not only explains the motion of the stars cor-
rectly, but also the field of force experienced by
himself.
' The field disappears at tlie centre of the disc and increases pro-
portionally to the distance from the centre as v7e proceed outwards.
BEHAVIOUR OF CLOCKS AND RODS 95
The observer performs experiments on his cir-
cular disc with clocks and measuring-rods. In
doing so, it is his intention to arrive at exact
definitions for the signification of time- and
space-data with reference to the circular disc K',
these definitions being based on his observations.
What wiU be his experience in this enterprise?
To start with, he places one of two identically
constructed clocks at the centre of the circular
disc, and the other on the edge of the disc, so that
they are at rest relative to it. We now ask our-
selves whether both clocks go at the same rate
from the standpoint of the non-rotating Galileian
reference-body K. As judged from this body,
the clock at the centre of the disc has no velocity,
whereas the clock at the edge of the disc is in
motion relative to iT in consequence of the rota-
tion. According to a result obtained in Section
XII, it follows that the latter clock goes at a rate
permanently slower than that of the clock at
the centre of the circular disc, i.e. as observed
from K. It is obvious that the same effect would
be noted by an observer whom we will imagine
sitting alongside his clock at the centre of the
circular disc. Thus on our circular disc, or, to
make the case more general, in every gravitational
field, a clock wiU go more quickly or less quickly,
according to the position in which the clock is
situated (at rest). For this reason it is not
96 GENERAL THEORY OF RELATIVITY
possible to obtain a reasonable definition of time
with the aid of docks which are arranged at
rest with respect to the body of reference. A
similar difficulty presents itself when we attempt
to apply our earlier definition of simultaneity in
such a case, but I do not wish to go any farther
into this question.
Moreover, at this stage the definition of the
space co-ordinates also presents xmsurmoimtable
difficulties. If the observer applies his standard
measuring-rod (a rod which is short as compared
with the radius of the disc) tangentiaUy to the
edge of the disc, then, as judged from the Galileian
system, the length of this rod wiU be less than i,
since, according to Section XII, moving bodies
suffer a shortening in the direction of the motion.
On the other hand, the measuring-rod wiU not
experience a shortening in length, as judged from
K, if it is applied to the disc in the direction of
the radius. If, then, the observer first measures
the circimiference of the disc with his measuring-
rod and then the diameter of the disc, on divid-
ing the one by the other, he wiU not obtain as
quotient the familiar number ir= 3.14 . . ., but
a larger niunber,^ whereas of course, for a disc
which is at rest with respect to K, this operation
' Throughout this consideration we have to use the Galildan
(non-Totating) system K as reference-body, since we may only assume
the validity of the results of the special theory of relativity relative
to K (relative to £' a gravitational field prevails).
BEHAVIOUR OF CLOCKS AND RODS 97
■would yield tt exactly. This proves that the
propositions of Euclidean geometry cannot hold
exactly on the rotating disc, nor in general in a
gravitational field, at least if we attribute the
length I to the rod in all positions and in every
orientation. Hence the idea of a straight line
also loses its meaning. We are therefore not in
a position to define exactly the co-ordinates
X, y, z relative to the disc by means of the method
used in discussing the special theory, and as long
as the co-ordinates and times of events have not
been defined we cannot assign an exact meaning
to the natural laws in which these occur.
Thus all our previous conclusions based on
general relativity would appear to be caUed in
question. In reality we must make a subtle
detour in order to be able to apply the postulate
of general relativity exactly. I shall prepare
the reader for this in the following paragraphs.
XXIV
EUCLIDEAN AND NON-EUCLIDEAN
CONTINUUM
THE surface of a marble table is spread out
in front of me. I can get from any one
point on this table to any other point by
passing continuously from one point to a "neigh-
boiuing" one, and repeating this process a (large)
niunber of times, or, in other words, by going
from point to point without executing jimips."
I am sure the reader will appreciate with sufficient
clearness what I mean here by "neighbouring"
and by " jmnps" (if he is not too pedantic). We
express this property of the surface by describing
the latter as a continuum.
Let us now imagine that a large number of
little rods of equal length have been made, their
lengths being smaU compared with the dimensions
of the marble slab. When I say they are of equal
length, I mean that one can be laid on any other
without the ends overlapping. We next lay four
of these Uttle rods on the marble slab so that they
constitute a quadrilateral figure (a square), the
diagonals of which are equally long. To ensure
the equality of the diagonals, we make use of a
EUCLIDEAN AND NON-EUCLIDEAN 99
little testing-rod. To this square we add similar
ones, each of which has one rod in common with
the first. We proceed in like manner with each of
these squares untU finally the whole marble slab
is laid out with squares. The arrangement is
such, that each side of a square belongs to two
squares and each comer to four squares.
It is a veritable wonder that we can carry out
this business without getting into the greatest
difficulties. We only need to think of the fol-
lowing. If at any moment three squares meet
at a comer, then two sides of the fourth square
are already laid, and as a consequence, the ar-
rangement of the remaining two sides of the
square is already completely determined. But
I am now no longer able to adjust the quadrilateral
so that its diagonals may be equal. If they are
equal of their own accord, then this is an especial
favour of the marble slab and of the little rods
about which I can only be thankfully surprised.
We must needs experience many such surprises
if the construction is to be successful.
If everything has really gone smoothly, then
I say that the points of the marble slab constitute a
Euclidean continuiun with respect to the little
rod, which has been used as a "distance" (line-
interval). By choosing one corner of a square as
"origin," I can characterise every other comer
of a square with reference to this origin by means
100 GENERAL THEORY OF RELATIVITY
of two numbers. I only need state how many
rods I must pass over when, starting from the
origin, I proceed towards the "right" and then
"upwards," in order to arrive at the comer of the
square under consideration. These two numbers
are then the "Cartesian co-ordmates" of this
comer with reference to the "Cartesian co-
ordinate system" which is determined by the
arrangement of Uttle rods.
By making use of the following modification
of this abstract experiment, we recognise that
there must also be cases in which the experiment
would be unsuccessf;il. We shall suppose that
the rods "expand" by an amoimt proportional to
the increase of temperature. We heat the central
part of the marble slab, but not the periphery,
in which case two of our Uttle rods can still be
brought into coincidence at every position on
the table. But our construction of squares must
necessarily come into disorder diuiug the heating,
because the Uttle rods on the central region of
the table expand, whereas those on the outer
part do not.
With reference to our Uttle rods — defined as
imit lengths — the marble slab is no longer a
EucUdean continuum, and we are also no longer
in the position of defining Cartesian co-ordinates
directly with their aid, since the above constmc-
tion can no longer be carried out. But since
EUCLroEAN AND NON-EUCLIDEAN 101
there are other things which are not influenced
in a similar manner to the little rods (or perhaps
not at aU) by the temperature of the table, it is
possible quite naturally to maintain the point of
view that the marble slab is a "EucHdean con-
tinuum." This can be done in a satisfactory
manner by making a more subtle stipulation
about the measurement or the comparison of
lengths.
But if rods of every kind {i.e. of every material)
were to behave in the same way as regards the
influence of temperature when they are on the
variably heated marble slab, and if we had no
other means of detecting the effect of temperature
than the geometrical behaviour of our rods in
experiments analogous to the one described above,
then our best plan woidd be to assign the distance
one to two points on the slab, provided that the
ends of one of our rods could be made to coincide
with these two points; for how else shoidd we
define the distance without our proceeding being
in the highest measure grossly arbitrary? The
method of Cartesian co-ordinates must then be
discarded, and replaced by another which does
not assume the vaUdity of Euclidean geometry
for rigid bodies.^ The reader will notice that
* Mathematicians have been confronted with our problem in the
following form. If we are given a surface (e.g. an ellipsoid) in Eucli-
dean three-dimensional space, then there exists for this surface a
two-dimensional geometry, just as much as for a plane surface.
102 GENERAL THEORY OF RELATIVITY
the situation depicted here corresponds to the one
brought about by the general postulate of relativity
(Section XXIII).
Gauss undertook the task of treating this two-dimensional geometry
from first principles, without making use of the fact that the surface
belongs to a Euclidean continuum of three dimensions. If we im-
agine constructions to be made with rigid rods in the surface (similar
to that above with the marble slab), we should find that different
laws hold for these from those resulting on the basis of Euclidean
plane geometry. The surface is not a Euclidean continuum with
respect to the rods, and we cannot define Cartesian co-ordinates in
the surface. Gauss indicated the principles according to which we
can treat the geometrical relationships in the surface, and thus
pointed out the way to the method of Riemann of treating multi-
dimensional, non-Euclidean continua. Thus it is that mathemati-
cians long ago solved the formal problems to which we are led by the
general postulate of relativity.
XXV
GAUSSIAN CO-ORDINATES
ACCORDING to Gauss, this combined ana-
lytical and geometrical mode of handling
the problem can be arrived at in the
following way. We imagine a system of arbitrary
ciurves (see Fig. 4) drawn, on the surface of the
table. These we designate as M-curves, and we
indicate each of them by means of a number.
The curves u=i, u=2 and u = ^ are drawn
in the diagram. Between the curves «= i and
u = 2we must imagine an
infinitely large nxmiber to
be drawn, aU of which
correspond to real niun-
bers lying between i and
2. We have then a system
of M-curves, and this "in- " '*'
finitely dense " systern covers the whole surface of
the table. These «<-curves must not intersect each
other, and through each point of the surface one
and only one curve must pass. Thus a perfectly
definite value of u belongs to every point on the
surface of the marble slab. In like manner we
103
104 GENERAL THEORY OP RELATIVITY
imagine a S5rstem of f-curves drawn on the surface.
These satisfy the same conditions as the «-curves,
they are provided with numbers in a correspond-
ing manner, and they may hkewise be of arbitrary
shape. It follows that a value of u and a value
of V belong to every point on the surface of the
table. We call these two numbers the co-or-
dinates of the surface of the table (Gaussian
co-ordinates). For example, the point P in the
diagram has the Gaussian co-ordinates m = 3,
v=\. Two neighbouring points P and P' on
the surface then correspond to the co-ordinates
P: u, V
P': u -{■ du, V -k- dv,
where du and dv signify very small numbers. In
a similar manner we may indicate the distance
(line-interval) between P and P', as measured
with a little rod, by means of the very small
number ds. Then according to Gauss we have
ds^ - f u du^ + 2gi2 du dv + gjz d'^,
where gn, gn, ga, are magnitudes which depend
in a perfectly definite way on u and v. The
magnitudes gn, gvt and ^22 determine the behaviour
of the rods relative to the ^-curves and c-curves,
and thus also relative to the smrface of the table.
For the case in which the points of the smiace
considered form a Euchdean continuum with
reference to the measuring-rods, but only in this
case, it is possible to draw the w-curves and
GAUSSIAN CO-ORDINATES 105
»-curves and to attach numbers to them, in such
a manner, that we simply have:
ds^ = du^ + dv^.
Under these conditions, the w-curves and j;-curves
are straight Hnes in the sense of EucHdean geom-
etry, and they are perpendicular to each other.
Here the Gaussian co-ordinates are simply Car-
tesian ones. It is clear that Gauss co-ordinates
are nothing more than an association of two sets
of numbers with the points of the surface con-
sidered, of such a nature that mmierical values
diEEering very sHghtly from each other are asso-
ciated with neighbouring points "in space."
So far, these considerations hold for a con-
tinuum of two dimensions. But the Gaussian
method can be applied also to a continuum of
three, four or more dimensions. If, for instance,
a continuum of four dimensions be supposed
available, we may represent it in the following
way. With every point of the continuum we
associate arbitrarily four numbers, Xi, 0C2, Xz, Xi,
which are known as "co-ordinates." Adjacent
points correspond to adjacent values of the co-
ordinates. If a distance ds is associated with
the adjacent points P and P', this distance being
measurable and well-defined from a physical point
of view, then the following formula holds:
di* -= gu dxi^ + 2gjidxidxt . . . . +gu dxi.
106 GENERAL THEORY OP RELATIVITY
where the magnitudes gn, etc., have values which
vary with the position in the continuum. Only
when the continuimi is a Euchdean one is it
possible to associate the co-ordinates Xi . . Xt with
the points of the continuum so that we have
simply
ds^ = dxi^ + dxi + dx^ + dx^.
In this case relations hold in the foiir-dimensional
continuimi which are analogous to those holding
in our three-dimensional measurements.
However, the Gauss treatment for ds^ which
we have given above is not always possible. It
is only possible when sufficiently small regions
of the continuum xmder consideration may be
regarded as Euclidean continua. For example,
this obviously holds in the case of the marble slab
of the table and local variation of temperature.
The temperature is practically constant for a
small part of the slab, and thus the geometrical
behaviour of the rods is almost as it ought to be
according to the ndes of Euchdean geometry.
Hence the imperfections of the construction of
squares in the previous section do not show them-
selves clearly imtil this construction is extended
over a considerable portion of the surface of the
table.
We can sum this up as follows: Gauss invented
a method for the mathematical treatment of
continua in general, in which "size-relations"
GAUSSIAN CO-ORDINATES 107
("distances" between neighbouring points) are
defined. To every point of a continuum are
assigned as many numbers (Gaussian co-ordi-
nates) as the continuum has dimensions. This
is done in such a way, that only one meaning can
be attached to the assignment, and that niunbers
(Gaussian co-ordinates) which differ by an in-
definitely small amount are assigned to adjacent
points. The Gaussian co-ordinate system is a
logical generaUsation of the Cartesian co-ordinate
system. It is also apphcable to non-Euchdean
continua, but only when, with respect to the
defined "size" or "distance," smaU parts of
the continuiun under consideration behave more
nearly like a Euclidean system, the smaller the
part of the continuum under our notice.
XXVI
THE SPACE-TIME CONTINUUM OF THE SPE-
CIAL THEORY OF RELATIVITY CONSID-
ERED AS A EUCLIDEAN CONTINUUM
WE are now in a position to formulate
more exactly the idea of Minkowski,
which was only vaguely indicated in
Section XVII. In accordance with the special
theory of relativity, certain co-ordinate systems
are given preference for the description of the
four-dimensional, space-time continuimi. We
called these "Galileian co-ordinate systems."
For these systems, the iova co-ordinates x, y,
z, t, which determine an event or — in other
words — a point of the four-dimensional con-
tinuvun, are defined physically in a simple maimer,
as set forth in detail in the first part of this book.
For the transition from one Galileian sjrstem to
another, which is moving imiformly with reference
to the first, the equations of the Lorentz trans-
formation are vaUd. These last form the basis
for the derivation of deductions from the special
theory of relativity, and in themselves they are
nothing more than the expression of the universal
108
SPACE-TIME CONTINUUM 109
validity of the law of transmission of light for all
Galileian systems of reference.
Minkowski foimd that the Lorentz transforma-
tions satisfy the following simple conditions.
Let us consider two neighbouring events, the
relative position of which in the four-dimensional
continuum is given with respect to a GaUleian
reference-body K by the space co-ordinate dif-
ferences dx, dy, dz and the time-difference dt.
With reference to a second Galileian system we
shall suppose that the corresponding differences
for these two events are dx', dy', dz', dt'. Then
these magnitudes always fulfil the condition.*
(ir^ + dy2 + dz^ -c'df^ dxf^ + dy"" + dz'^ - c" dt'\
The validity of the Lorentz transformation
follows from this condition. We can express this
as follows: The magnitude
ds^ =dx' + dy^ -I- dz' - c^ df,
which belongs to two adjacent points of the four-
dimensional space-time continuiun, has the same
value for all selected (GaUleian) reference-bodies.
If we replace x, y, z, ■\/~^ ct, by Xi, Xi, Xz, Xi, we
also obtain the result that
ds^ = dxi^ + dxi^ + dxi^ + dxi^
is independent of the choice of the body of refer-
• Cf. Appendices I and 11. The relations which are derived
there for the co-ordinates themselves are valid also for co-ordinate
diferences, and thus also for co-ordinate difEerentials (indefinitely-
small differences).
110 GENERAL THEORY OF RELATIVITY
ence. We call the magnitude ds the "distance"
apart of the two events or four-dimensional points.
Thus, if we choose as time-variable the im-
aginary variable ■>/ — i ct instead of the real
quantity t, we can regard the space-time con-
tinuum — in accordance with the special theory
of relativity — as a "Euclidean" four-dimensional
continuum, a result which foUows from the
considerations of the preceding section.
xxvn
THE SPACE-TIME CONTINUUM OF THE
GENERAL THEORY OF RELATIVITY IS
NOT A EUCLIDEAN CONTINUUM
IN the first part of this book we were able to
make use of space-time co-ordinates which
allowed of a simple and direct physical in-
terpretation, and which, according to Section
XXVI, can be regarded as four-dimensional
Cartesian co-ordinates. This was possible on
the basis of the law of the constancy of the ve-
locity of Hght. But according to Section XXI,
the general theory of relativity cannot retain
this law. On the contrary, we arrived at the
result that according to this latter theory the
velocity of light must always depend on the co-
ordinates when a gravitational field is pres-
ent. In cormection with a specific illustration in
Section XXIII, we found that the presence of
a gravitational field invahdates the definition of
the co-ordinates and the time, which led us to
our objective in the special theory of relativity.
In view of the results of these considerations
we are led to the conviction that, according to
111
112 GENERAL THEORY OF RELATIVITY
the general principle of relativity, the space-time
continuum cannot be regarded as a Euclidean
one, but that here we have the general case,
corresponding to the marble slab with local
variations of temperature, and with which we
made acquaintance as an example of a two-
dimensional continuiun. Just as it was there
impossible to construct a Cartesian co-ordinate
system from equal rods, so here it is impossible
to build up a system (reference-body) from rigid
bodies and clocks, which shall be of such a nature
that measuring-rods and clocks, arranged rigidly
with respect to one another, shall indicate posi-
tion and time directly. Such was the essence of
the difl&culty with which we were confronted in
Section XXIII.
But the considerations of Sections XXV and
XXVI show us the way to surmount this difl&-
culty. We refer the four-dimensional space-time
continuum in an arbitrary manner to Gauss
co-ordinates. We assign to every point of the
continuiun (event) four mambers, Xi, %, Xt, Xt
(co-ordinates), which have not the least direct
physical significance, but only serve the purpose
of numbering the points of the continuum in a
definite but arbitrary manner. This arrangement
does not even need to be of such a kind that we
must regard Xi, X2, Xs, as "space" co-ordinates
and Xi as a "time" co-ordinate.
SPACE-TIME CONTINUUM 113
The reader may think that such a description
of the world would be quite inadequate. What
does it mean to assign to an event the particular
co-ordinates Xi, od, %, x^, if in themselves these
co-ordinates have no significance? More careful
consideration shows, however, that this anxiety-
is imfoimded. Let us consider, for instance, a
material point with any kind of motion. If this
point had only a momentary existence without
duration, then it woiild be described in space-
time by a single system of values Xi, oc^, Xs, Xi.
Thus its permanent existence must be char-
acterised by an infinitely large nimiber of such
systems of values, the co-ordinate values of
which are so close together as to give continuity;
corresponding to the material point, we thus have
a (uni-dimensional) line in the four-dimensional
continuima. In the same way, any such lines
in oiur continuum correspond to many points in
motion. The only statements having regard to
these points which can claim a physical existence
are in reality the statements about their en-
counters. In our mathematical treatment, such
an encovmter is expressed in the fact that the
two lines which represent the motions of the
points in question have a particular system of
co-ordinate values, Xi, %, Xz, X4, in common.
After mature consideration the reader will doubt-
less admit that in reality such encounters con-
114 GENERAL THEORY OF RELATIVITY
stitute the only actual evidence of a time-space
nature with which we meet in physical statements.
When we were describing the motion of a
material ppint relative to a body of reference, we
stated nothing more than the encoxmters of this
point with particular points of the reference-body.
We can also determine the correspondiag values
of the time by the observation of encounters of
the body with clocks, in conjunction with the
observation of the encounter of the hands of
clocks with particular points on the dials. It is
just the same in the case of space-measurements
by means of measuring-rods, as a Uttle considera-
tion will show.
I The following statements hold generally: Every
physical description resolves itself into a number
of statements, each of which refers to the space-
time coincidence of two events A and B. In
terms of Gaussian co-ordinates, every such state-
ment is expressed by the agreement of their four
co-ordinates Xi, %, %, Xi. Thus in reality, the
description of the time-space continuum by
means of Gauss co-ordinates completely replaces
the description with the aid of a body of reference,
without suffering from the defects of the latter
mode of description; it is not tied down to the
Euclidean character of the continuum which has
to be represented.
xxvm
EXACT FORMULATION OF THE GENERAL
PRINCIPLE OF RELATIVITY
WE are now in a position to replace the
provisional formulation of the general
principle of relativity given in Section
XVIII by an exact formulation. The form
there used, "All bodies of reference K, K', etc.,
are equivalent for the description of natural
phenomena (formulation of the general laws of
nature), whatever may be their state of motion,"
cannot be maintained, because the use of rigid
reference-bodies, in the sense of the method fol-
lowed in the special theory of relativity, is in
general not possible in space-time description.
The Gauss co-ordinate system has to take the
place of the body of reference. The following
statement corresponds to the fundamental idea
of the general principle of relativity: "All Gaus-
sian co-ordinate systems are essentially equivalent
for the formulation of the general laws of nature."
We can state this general principle of relativity
in stiU another form, which renders it yet more
clearly intelligible than it is when in the form of
H5j
116 GENERAL THEORY OF RELATIVITY
the natural extension of the special principle
of relativity. According to the special theory of
relativity, the equations which express the general
laws of nature pass over into equations of the
same form when, by making use of the Lorentz
transformation, we replace the space-time variables
X, y, z, t, of a (Galileian) reference-body K by the
space-time variables x', y', z', t', of a new reference-
body K'. According to the general theory of
relativity, on the other hand, by appUcation of
arbitrary substitutions of the Gauss variables Xi,
Xi, Xz, Xi, the equations must pass over into
equations of the same form; for every transfor-
mation (not only the Lorentz transformation)
corresponds to the transition of one Gauss co-ordi-
nate system into another.
If we desire to adhere to our "old-time" three-
dimensional view of things, then we can char-
acterise the development which is being imder-
gone by the fundamental idea of the general
theory of relativity as follows: The special theory
of relativity has reference to Galileian domains,
i.e. to those in which no gravitational field exists.
In this connection a Galileian reference-body
serves as body of reference, i.e. a rigid body the
state of motion of which is so chosen that the
Galileian law of the imifonn rectilinear mo-
tion of "isolated" material points holds relatively
to it.
GENERAL PRINCIPLE OF RELATIVITY 117
Certain considerations suggest that we should
refer the same Galileian domains to non-Galileian
reference-bodies also. A gravitational field of a
special kind is then present with respect to these
bodies (cf. Sections XX and XXIII).
In gravitational fields there are no such things
as rigid bodies with Euclidean properties; thus
the fictitious rigid body of reference is of no avail
in the general theory of relativity. The motion
of clocks is also influenced by gravitational fields,
and in such a way that a physical definition of
time which is made directly with the aid of clocks
has by no means the same degree of plausibility
as in the special theory of relativity.
For this reason non-rigid reference-bodies are
used which are as a whole not only moving in any
way whatsoever, but which also suffer alterations
in form ad lib. during their motion. Clocks, for
which the law of motion is of any kind, however
irregular, serve for the definition of time. We
have to imagine each of these clocks fixed at a point
on the non-rigid reference-body. These clocks
satisfy only the one condition, that the "readings"
which are observed simultaneously on adjacent
clocks (in space) differ from each other by an
indefinitely small amoimt. This non-rigid refer-
ence-body, which might appropriately be termed
a "reference-moUusk," is in the main equivalent
to a Gaussian four-dimensional co-ordinate sys-
118 GENERAL THEORY OF RELATIVITY
tern chosen arbitrarily. That which gives the
"mollusk" a certain comprehensibleness as com-
pared with the Gauss co-ordinate system is the
(really unjustified) formal retention of the sep-
arate existence of the space co-ordinates as op>-
posed to the time co-ordinate. Every point on
the moUusk is treated as a space-point, and every
material point which is at rest relatively to it as
at rest, so long as the mollusk is considered as
reference-body. The general principle ^ f rela;:^
tivity requires that all these^mpllu^isxanJiejLised
as fefefencejodies jmti__egiial right and equal
success in the formulation^f_thg^general laws-oi
natilfe- '^the laws themselves must be quite
indepaident of the choice^ moUusk.
The--gr-ea£^Hpgwig"f>9sses&idl3Bc:die general
principle of relativi ty hes in the comprehe nsive
limitation which is imposed on the laws of nature
in consequence of what we have seen^OMCZi; —
xxrx
THE SOLUTION OF THE PROBLEM OF GRAVI-
TATION ON THE BASIS OF THE GENERAL
PRINCIPLE OF RELATIVITY
IF the reader has followed all our previous
considerations, he will have no further diffi-
culty in understanding the methods leading
to the solution of the problem of gravitation.
We start off from a consideration of a Gahleian
domain, i.e. a domain in which there is no gravita-
tional field relative to the Galileian reference-
body K. The behaviour of measuring-rods and
clocks with reference to K is known from the
special theory of relativity, likewise the behaviour
of "isolated" material points; the latter move
uniformly and in straight lines.
Now let us refer this domain to a random Gauss
co-ordinate system or to a "mollusk" as reference-^
body K'. Then with respect to K' there is a,
gravitational field G (of a particular kind). We
learn the behaviour of measuring-rods and clocks
and also of freely-moving material points with
reference to K' simply by mathematical trans-
formation. We interpret this behaviour as the
119
120 GENERAL THEORY OF RELATIVITY
behaviour of measuring-rods, clocks and material
points under the influence of the gravitational
field G. Hereupon we introduce a hypothesis:
that the influence of the gravitational field on
measuring-rods, clocks and freely-moving material
points continues to take place according to the
same laws, even in the case when the prevailing
gravitational field is not derivable from the
GaUleian special case, simply by means of a
transformation of co-ordinates.
The next step is to investigate the space-time
behaviour of the gravitational field G, which was
derived from the GaHleian special case simply by
transformation of the co-ordinates. This be-
haviour is formulated in a law, which is always
valid, no matter how the reference-body (mollusk)
used in the description may be chosen.
This law is not yet the general law of the gravita-
tional field, since the gravitational field imder
consideration is of a special kind. In order to
find out the general law-of -field of gravitation we
still require to obtain a generalisation of the law
as found above. This can be obtained without
caprice, however, by taking into consideration
the following demands:
(a) The required generalisation must likewise
satisfy the general postulate of relativity.
(b) If there is any matter in the domain under
consideration, only its inertial masSj and
SOLUTION OF GRAVITATION 121
thus according to Section XV only its
energy is of importance for its effect in
exciting a field,
(c) Gravitational field and matter together
must satisfy the law of the conservation
of energy (and of impulse).
Finally, the general principle of relativity per-
mits us to determine the influence of the gravita-
tional field on the course of all those processes
which take place according to known laws when a
gravitational field is absent, i.e. which have
already been fitted into the frame of the special
theory of relativity. In this connection we pro-
ceed in principle according to the method which
has already been explained for measuring-rods,
clocks and freely-moving material points.
The th eory of gr avitation derived in th l&-way
from the general postulate of relativity^ excels..
not only in its beautyjlttor in removing the defect
attachih^tb classic al mechanic s which was'brought
to light in Section XXIj nor in interpret^TEe
empirical law of the equality~br~mertial and
gravitational mass; but it has also already ex-
plained a result of observation in astronomy,
against -which classical mechanics is powerless.
If we confine the application of the theory to
the case where the gravitational fields can be
regarded as being weak, and in which all masses
move with respect to the co-ordinate system with
122 GENERAL THEORY OF RELATIVITY
velocities which are small compared with the
velocity of light, we then obtain as a first ap-
proximation the Newtonian theory. Thus the
latter theory is obtained here without any particu-
lar assumption, whereas Newton had to iatroduce
the hypothesis that the force of attraction between
mutually attracting material points is inversely
proportional to the square of the distance between
them. If we increase the accuracy of the calcu-
lation, deviations from the theory of Newton
make their appearance, practically all of which
must nevertheless escape the test of observation
owing to their smaUness.
We must draw attention here to one of these
deviations. According to Newton's theory, a
planet moves round the sun in an ellipse, which
would permanently maintain its position with
respect to the fixed stars, if we could disregard
the motion of the fixed stars themselves and the
action of the other planets under consideration.
Thus, if we correct the observed motion of the
planets for these two influences, and if Newton's
theory be strictly correct, we ought to obtain
for the orbit of the planet an ellipse, which is
fixed with reference to the fixed stars. This
deduction, which can be tested with great ac-
curacy, has been confirmed for all the planets
save one, with the precision that is capable of
being obtained by the delicacy of observation
SOLUTION OF GRAVITATION 123
attainable at the present time. The sole ex-
ception is Mercury, the planet which hes nearest
the sun. Since the time of Leverrier, it has been
known that the ellipse corresponding to the orbit
of Mercury, after it has been corrected for the
influences mentioned above, is not stationary with
respect to the fixed stars, but that it rotates ex-
ceedingly slowly in the plane of the orbit and ia
the sense of the orbital motion. The value ob-
tained for this rotary movement of the orbital
ellipse was 43 seconds of arc per century, an
amount ensured to be correct to within a few
seconds of arc. This effect can be explained by
means of classical mechanics only on the as-
sumption of hypotheses which have Uttle proba-
bility, and which were devised solely for this
purpose.
On the basis of the general theory of relativity,
it is found that the eUipse of every planet roimd
the sun must necessarily rotate in the manner
indicated above; that for all the planets, with
the exception of Mercury, this rotation is too
small to be detected with the delicacy of ob-
servation possible at the present time; but that
in the case of Mercury it must amount to 43
seconds of arc per century, a result which is strictly
in agreement with observation.
Apart from this one, it has hitherto been possible
to make only two deductions from the theory
124 GENERAL THEORY OF RELATIVITY
which admit of being tested by observation, to wit,
the curvature of light rays by the gravitational
field of the sun/ and a displacement of the spectral
lines of Ught reaching us from large stars, as com-
pared with the corresponding lines for Hght pro-
duced in an analogous manner terrestrially {i.e.
by the same kind of molecule). I do not doubt
that these deductions from the theory wiU be'
confirmed also.
' Observed by Eddington and others in igig. (Cf. Appendix
m.)
PART III
CONSIDERATIONS ON THE UNIVERSE
AS A WHOLE
XXX
COSMOLOGICAL DIFFICULTIES OF NEWTON'S
THEORY
APART from the difl&culty discussed in Sec-
tion XXI, there is a second fundamental
difficulty attending classical celestial me-
chanics, which, to the best of my knowledge,
was first discussed in detail by the astronomer
Seeliger. If we ponder over the question as to
how the universe, considered as a whole, is to be
regarded, the first answer that suggests itself to
us is surely this: As^egards_j2ace_(and time)
thejmiyersejs_jnfiiiite. There are stars^evH^^
where, so that the density of matter, although
very variable in detail, is nevertheless on the
average everywhere the same. In other words:
However far we might travel through space, we
shoiild find ever3rwhere an attenuated swarm of
fixed stars of approximately the same kind and
density.
125
126 CONSIDERATIONS ON THE UNIVERSE
This view is not in harmony with the theory of
Newton. The latter theory rather requires that
the universe should have a kind of centre in which
the density of the stars is a maximimi, and that
as we proceed outwards from this centre the
group-density of the stars should diminish, imtil
finally, at great distances, it is succeeded by an
infinite region of emptiness. Hie^^ar univ^se^
ought to be a finite island in ^le^infinite ocea^jrf
space.^
This conception is in itself not very satisfactory.
It is still less satisfactory becauselTTeads^to the
result that the light emitted by the stars and also
individual stars of the stellar system are per-
petually passing out into infinite space, never
to return, and without ever again coming into
interaction with other objects of nature. ^^_
a finite material universe woul d be de stmgd
to become gradually but systematically^ impQv=_
erished.
1 Proof. — According to the theory of Newton, the number of
'.'lines of force" which come from infinity and terminate in a mass
m is proportional to the mass m. K, on the average, the mass-den-
sity po is constant throughout the imiverse, then a sphere of volume
V will enclose the average mass poF. Thus the number of lines of
force passing through the surface F of the sphere into its interior is
proportional to poK. For vmit area of the surface of the sphere the
nurlber of lines of force which enters the sphere is thus proportional
V
to popOT to peR. Hence the intensity of the field at the surface would
ultimately become infinite with increasing radius R of the sphere,
which is impossible.
NEWTON'S THEORY 127
In order to escape this dilemma, Seeliger sug-
gested a modification of Newton's law, in which
he assumes that for great distances the force of
attraction between two masses diminishes more
rapidly than would result from the inverse square
law. In this way it is possible for the mean
density of matter to be constant everywhere, even
to infinity, without infinitely large gravitational
fields being produced. We thus free ourselves
from the distasteful conception, that the material
universe ought to possess something of the nature
of a centre. Of course we purchase our.emancipa-
tionjfcom the fimdamental difficulties mentioned,
at the cost of a^modification-and-comphcatign of
Newton's law which has neither empirical nor
theoretical foimdation. We can imagine innum-
erable laws which would serve the same piupose,
without our being able to state a reason why one
of them is to be preferred to the others; for any
one of these laws would be foimded just as little
on more general theoretical principles as is the
law of Newton.
XXXI
THE POSSIBILITY OF A "FINITE" AND YET
"UNBOUNDED" UNIVERSE
BUT speculations on the structure of the
universe also move in quite another direc-
tion. The development of non-Euclidean
geometry led to the recognition of the fact,
that we can cast doubt on the infiniteness of
our space without coming into conflict with the
laws of thought or with experience (Riemann,
Hehnholtz). These questions have already been
treated in detail and with unsurpassable lucidity
by Helmholtz and Poincare, whereas I can only
touch on them briefly here.
In the first place, we imagiae an existence in
two-dimensional space. Flat beings with flat
implements, and in particular flat rigid measuring-
rods, are free to move in a plane. For them
nothing exists outside of this plane: that which
they observe to happen to themselves and to their
flat "things" is the aU-rnclusive reaUty of their
plane. In particular, the constructions of plane
Euclidean geometry can be carried out by means
of the rods, e.g. the lattice construction, con-
M8
UNIVERSE — FINITE YET UNBOUNDED 129
sidered in Section XXIV. In contrast to ours,
the universe of these beings is two-dimensional;
but, Uke ours, it extends to infinity. In their
universe there is room for an infinite number of
identical squares made up of rods, i.e. its volume
(surface) is infinite. If these beings say their
universe is "plane," there is sense in the state-
ment, because they mean that they can perform
the constructions of plane Euclidean geometry
with their rods. In this cormection the indi-
vidual rods always represent the same distance,
independently of their position.
Let us consider now a second two-dimensional
existence, but this time on a spherical sxirface
instead of on a plane. The flat beings with their
measuring-rods and other objects fit exactly on
this surface and they are imable to leave it. Their
whole universe of observation extends exclusively
over the surface of the sphere. Are these beings
able to regard the geometry of their universe as
being plane geometry and their rods withal as
the realisation of "distance"? They cannot do
this. For if they attempt to realise a straight
line, they will obtain a curve, which we "three-
dimensional beings" designate as a great circle,
i.e. a self-contained line of definite finite length,
which can be measured up by means of a measur-
ing-rod. Similarly, this universe has a finite
area, that can be compared with the area of a
130 CONSIDERATIONS ON THE UNIVERSE
square constructed with rods. Th e great cha rm
resulting^^from iiris^-consideration ji es in the
recognition oljlie^facl-tliat the universe_j;^.Jhes€-~
b^ngsJs^mte'and'yethasno limits.
But the spherical-surface beings do not need
to go on a world-tour in order to perceive that they
are not living in a EucUdean universe. They can
convince themselves of this on every part of their
"world," provided they do not use too small a
piece of it. Starting from a point, they draw
"straight lines" (arcs of circles as judged in
three-dimensional space) of equal length in aU
directions. They wiU call the line joining the
free ends of these lines a "circle." For a plane
surface, the ratio of the circumference of a circle
to its diameter, both lengths being measvu-ed with
the same rod, is, according to Euclidean geometry
of the plane, equal to a constant value ir, which is
independent of the diameter of the circle. On
their spherical smrface our flat beings would find
for this ratio the value
sin
IT
©
(i)
i.e. a smaller value than ir, the difference being
the more considerable, the greater is the radius
of the circle in comparison with the radius R of
the "world-sphere." By means of this relation
UNIVERSE— FINITE YET UNBOUNDED 131
the spherical bemgs can determine the radius of
their universe ("world"), even when only a
relatively small part of their world-sphere is
available for their measurements. But if this
part is very small indeed, they will no longer be
able to demonstrate that they are on a spherical
"world" and not on a EucUdean plane, for a
small part of a spherical surface differs only sUghtly
from a piece of a plane of the same size.
Thus if the spherical-surface beings are living
on a planet of which the solar system occupies
only a negligibly small part of the spherical
universe, they have no means of determining
whether they are Uving in a finite or in an infinite
universe, because the "piece of universe" to
which they have access is in both cases prac-
tically plane, or Euclidean. It follows directly
from this discussion, that for our sphere-beings
the circimaference of a circle first increases with
the radius until the "circumference of the uni-
verse" is reached, and that it thenceforward
gradually decreases to zero for still further in-
creasing values of the radius. During this process
the area of the circle continues to increase more
and more, until finally it becomes equal to the
total area of the whole "world-sphere."
Perhaps the reader will wonder why we have
placed our "beings" on a sphere rather than on
another closed surface. But this choice has its
132 CONSIDERATIONS ON THE UNIVERSE
justification in the fact that, of all closed sur-
faces, the sphere is unique in possessing the
property that all points on it are equivalent. I
admit that the ratio of the circumference c of a
circle to its radius r depends on r, but for a given
value of r it is the same for all points of the
"world-sphere"; in other words, the "world-
sphere" is a "surface of constant curvature."
To this two-dimensional sphere-universe there
is *a three-dimensional analogy, namely, the
three-dimensional spherical space which was dis-
covered by Riemann. Its points are likewise all
equivalent. It possesses a finite volxmie, which
is determined by its "radius" (sTr^'i?'). Is it pos-
sible to imagine a spherical space? To imagine
a space means nothing else than that we imagine
an epitome of ovx "space" experience, i.e. of
experience that we can have in the movement of
"rigid" bodies. In this sense we can imagine
a spherical space.
Suppose we draw lines or stretch strings in aU
directions from a point, and mark oS from each
of these the distance r with a measuring-rod.
AU the free end-points of these lengths he on a
spherical surface. We can specially measure up
the area (F) of this surface by means of a square
made up of measuring-rods. If the universe is
Euclidean, then F= /\.'irr^; if it is spherical, then
F is always less than 47rr^. With increasing values
UNIVERSE— FINITE YET UNBOUNDED 133
of r, F increases from zero up to a maximum value
which is determined by the "world-radius," but
for still further increasing values of r, the area
gradually diminishes to zero. At first, the straight
lines which radiate from the starting point diverge
farther and farther from one another, but later
they approach each other, and finally they run
together again at a "counter-point" to the start-
ing point. Under such conditions they have
traversed the whole spherical space. It is easily
seen that the three-dimensional spherical space
is quite analogous to the two-dimensional spherical
surface. It is finite {i.e. of finite volume), and
has no bounds.
It may be mentioned that there is yet another
kind of curved space: "eUiptical space." It can
be regarded as a curved space in which the two
" coimter-points " are identical (indistinguishable
from each other). An elliptical imiverse can thus
be considered to some extent as a curved universe
possessing central symmetry.
It follows from what has been said, that closed
spaces without limits are conceivable. From
amongst-these, the spherical space (and the el-
liptical) excels in its simplicity, since all points on
it are_equivalent. As a result of this discussion,
a most interesting question arises for astronomers
and physicists, and that is whether the universe
in which we live is infinite, or whether it is finite
134 CONSIDERATIONS ON THE UNIVEBSE
in the maimer of the spherical universe. Our ex-
perience is far from being sufficient to enable us
to answer this question. But the general theory
of relativity permits of our answering it with a
moderate degree of certainty, and ia this con-
nection the difficulty mentioned in Section XXX
finds its solution.
xxxn
THE STRUCTURE OF SPACE ACCORDING TO
THE GENERAL THEORY OF RELATIVITY
ACCORDING to the general theory of
relativity, the geometrical properties of
space are not independent, but they are
determined by matter. Thus we can draw con-
clusions about the geometrical structure of the
tmiverse only if we base our considerations on
the state of the matter as being something that
is known. We know from experience that, for a
suitably chosen co-ordinate system, the velocities
of the stars are small as compared with the
velocity of transmission of light. We can thus
as a rough approximation arrive at a conclusion
as to the nature of the universe as a whole, if
we treat the matter as being at rest.
We aheady know from our previous discussion
that the behaviour of measuring-rods and clocks
is influenced by gravitational fields, i.e. by the
distribution of matter. This in itself is sufficient
to exclude the possibility of the exact vaUdity of
Euclidean geometry in our universe. But it is
conceivable that our imiverse diEEers only slightly
135
136 CONSroERATIONS ON THE UNIVERSE
from a Euclidean one, and this notion seems all
the more probable, since calculations show that
the metrics of surrounding space is influenced
only to an exceedingly small extent by masses
even of the magnitude of our sun. We might
imagine that, as regards geometry, our imiverse
behaves analogously to a surface which is ir-
regularly curved in its individual parts, but which
nowhere departs appreciably from a plane: some-
thing like the rippled surface of a lake. Such a
universe might fittingly be called a quasi-Eu-
clidean imiverse. As regards its space it would
be infinite. But calculation shows that in a
quasi-Euclidean imiverse the average density of
matter would necessarily be nil. Thus such a
universe could not be inhabited by matter every-
where; it would present to us that imsatisfactory
picture which we portrayed in Section XXX.
If we are to have in the universe an average
density of matter which differs from zero, how-
ever small may be that difference, then the
universe cannot be quasi-Euclidean. On the con-
trary, the results of calculation indicate that if
matter be distributed tmiformly, the imiverse
would necessarily be spherical (or elliptical).
Since in reality the detailed distribution of matter
is not uniform, the real imiverse will deviate in
individual parts from the spherical, i.e. the uni-
verse will be quasi-spherical. But it wiU be
THE STRUCTURE OF SPACE 137
necessarily finite. In fact, the theory suppHes
us with a simple connection ' between the space-
expanse of the universe and the average density
of matter in it.
'■ For the "radius" R of the universe we obtain the equation
Kp
2
The use of the C.G.S. system in this equation gives - = i-oS.io^';
p is the average density of the matter.
APPENDIX I
SIMPLE DERIVATION OF THE LORENTZ
TRANSFORMATION [Supplementary to Sec-
tion XI]
FOR the relative orientation of the co-ordi-
nate systems indicated in Fig. 2, the
aj-axes of both systems permanently co-
incide. In the present case we can di\dde the
problem into parts by considering first only
events which are locaUsed on the a;-axis. Any
such event is represented with respect to the co-
ordinate system K by the abscissa x and the
time t, and with respect to the system K' by the
abscissa x' and the time t'. We require to find
x' and /' when x and t are given.
A light-signal, which is proceeding along the
positive axis of x, is transmitted according to the
equation
x = ct
or
X- ct = O (l).
Since the same light-signal has to be transmitted
relative to K' with the velocity c, the propagation
ISO
140 APPENDIX I
relative to the system K' will be represented by
the analogous formula
x' -d = o (2).
Those space-time points (events) which satisfy
(i) must also satisfy (2). Obviously this wiU be
the case when the relation
{x'-ct')=\{x-ct) (3)
is fulfilled in general, where X indicates a con-
stant; for, according to (3), the disappearance
of {x — ct) involves the disappearance of (a;' — ct').
If we apply quite similar considerations to light
rays which are being transmitted along the
negative a;-axis, we obtain the condition
{x' + ct^ =fi(x + ct) (4).
By adding (or subtracting) equations (3) and (4),
and introducing for convenience the constants a
and b in place of the constants X and n where
X + M
a =
2
we obtain the equations
x' = ax — bct\ ^»
cf = act-bxi «^-
We should thus have the solution of our prob-
lem, if the constants a and b were known. These
result from the following discussion.
For the origin of K' we have permanently
x' = o, and hence according to the first of the
equations (5)
THE LORENTZ TRANSFORMATION 141
he
X = —t.
a
If we call V the velocity with which the origin
of K' is moviag relative to K, we then have
"1 w-
The same value v can be obtained from equa-
tion (5), if we calculate the velocity of another
point of K' relative to K, or the velocity (di-
rected towards the negative a;-axis) of a point of
K with respect to K\ In short, we can designate
V as the relative velocity of the two systems.
Furthermore, the principle of relativity teaches
us that, as judged from K, the length of a imit
measuring-rod which is at rest with reference to
K' must be exactly the same as the length, as
judged from K', of a imit measuring-rod which
is at rest relative to K. In order to see how the
points of the x'-axis appear as viewed from K,
we only require to take a "snapshot" of K' from
K; this means that we have to insert a particular
value of t (time of K), e.g. / = o. For this value of
/ we then obtain from the first of the equations (5)
x' = ax.
Two points of the x'-axis which are separated
by the distance Ax' = 1 when measured in the
K' system are thus separated in our instantaneous
photograph by the distance
Aa; = ^ (7).
14S
APPENDIX I
But if the snapshot be taken from K'^f^o),
and if we eliminate t from the equations (5),
taking into account the expression (6), we
obtain
From this we conclude that two points on the
a;-axis and separated by the distance i (relative to
K) will be represented on our snapshot by the
distance
Aio'-=a(i--^ (7fl).
But from what has been said, the two snap-
shots must be identical; hence A* in (7) must
be equal to Aa;' in (7a), so that we obtain
a^ = -^, • . (76).
The equations (6) and (7&) determine the con-
stants a and b. By inserting the values of these
constants in (5), we obtain the first and the
fourth of the equations given in Section XI.
0/-=
f~
X -
■vt
^
t-
V
^
v'
(8).
THE LORENTZ TRANSFORMATION 143
Thus we have obtained the Lorentz trans-
formation for events on the aj-axis. It satisfies
the condition
a;'2 _ cV2 = a;2 - c«^ (8a).
The extension of this result, to include events
which take place outside the a;-axis, is obtained by
retaining equations (8) and supplementing them
by the relations
i:i] w-
In this way we satisfy the postulate of the con-
stancy of the velocity of light in vacuo for rays
of Hght of arbitrary direction, both for the system
K and for the system K'. This may be shown in
the following manner.
We suppose a light-signal sent out from the
origin of iT at the time t= o. It will be propa-
gated according to the equation
r = -s/x^ + y^ + z' => ct,
or, if we square this equation, according to the
equation
s? + f + z*-c*^~o (lo).
It is required by the law of propagation of light,
in conjimction with the postulate of relativity,
that the transmission of the signal in question
should take place — as judged from K' — in
accordance with the corresponding formula
/ = d'
or,
a/2 4- y'« + s'2 _ CV« = O .... (lOffl).
144 APPENDIX I
In order that equation (loo) may be a consequence
of equation (lo), we must have
*'« + y"' + z'" - c^/'' = ffCr! + ys + z« - c«^) (ii).
Since equation (8a) must hold for points on the
jc-axis, we thus have a = i. It is easily seen
that the Lorentz transformation really satisfies
equation (ii) for o-= i; for (ii) is a consequence
of (8a) and (9), and hence also of (8) and (9).
We have thus derived the Lorentz transformation.
The Lorentz transformation represented by
(8) and (9) stiU requires to be generaUsed. Ob-
viously it is immaterial whether the axes of K'
be chosen so that they are spatially parallel to
those of K. It is also not essential that the
velocity of translation of K' with respect to K
should be in the direction of the a;-axis. A simple
consideration shows that we are able to construct
the Lorentz transformation in this general sense
from two kinds of transformations, viz. from
Lorentz transformations in the special sense and
from purely spatial transformations, which cor-
responds to the replacement of the rectangular
co-ordinate system by a new system with its
axes pointing in other directions.
Mathematically, we can characterise the gen-
eralised Lorentz transformation thus:
It expresses x', y', z' , t', in terms of linear
homogeneous functions of x, y, z, t, of such a kind
that the relation
THE LORENTZ TRANSFORMATION 145
is satisfied identically. That is to say: If we
substitute their expressions in x, y, z, t, in place of
x', y', z', t', on the left-hand side, then the left-
hand side of (no) agrees with the right-hand side.
APPENDIX II
MINKOWSKI'S FOUR — DIMENSIONAL SPACE
("WORLD") [Supplementary to Section XVII]
WE can characterise the Lorentz trans-
formation still more simply if we in-
troduce the imaginary V — i. ct in place
of t, as time-variable. If, in accordance with
this, we insert
Xi " y
xz " z
Xi - V— i.d,
and similarly for the accented system K', then the
condition which is identically satisfied by the
transformation can be expressed thus:
xi'^ + %'" + x»'^ + ««'« - Xi' + xa^ + x^ + x^. (12).
That is, by the afore-mentioned choice of "co-
ordinates " (iia) is transformed into this equation.
We see from (12) that the imaginary time co-
ordinate Xi enters into the condition of trans-
formation in exactly the same way as the space
co-ordinates Xi, %, Xz. It is due to this fact that,
according to the theory of relativity, the "time"
116
FOUR-DIMENSIONAL SPACE 147
Xi enters into natural laws in the same fonn as
the space co-ordinates Xi, x^, x^.
A four-dimensional continuum described by the
"co-ordinates" Xi, %, x^, Xi, was called "world"
by Minkowski, who also termed a point-event a
"world-point." From a "happening" in three-
dimensional space, physics becomes, as it were,
an "existence" in the four-dimensional "world."
This four-dimensional "world" bears a close
similarity to the three-dimensional "space" of
(Euclidean) analytical geometry. If we intro-
duce into the latter a new Cartesian co-ordinate
system {x\, x'2, x'3) with the same origin, then
x\, x'z, x'z, are linear homogeneous functions of
Xi, Xi, Xs, which identically satisfy the equation
Xi'^ + Xi'^ + JCs" = Xi^ + X2^ + xi.
The analogy with (12) is a complete one. We
can regard Minkowski's "world" in a formal
manner as a four-dimensional EucUdean space
(with imaginary time co-ordinate); the Lorentz
transformation corresponds to a "rotation" of
the co-ordinate system in the four-dimensional
"world."
APPENDIX III
THE EXPERIMENTAL CONFIRMATION OF THE
GENERAL THEORY OF RELATIVITY
FROM a systematic theoretical point of
view, we may imagine the process of
evolution of an empirical science to be a
continuous process of induction. Theories are
evolved, and are expressed in short compass as
statements of a large number of individual ob-
servations in the form of empirical laws, from
which the general laws can be ascertained by
comparison. Regarded in this way, the develop-
ment of a science bears some resemblance to the
compilation of a classified catalogue. It is, as
it were, a purely empirical enterprise.
But this point of view by no means embraces
the whole of the actual process; for it slurs over
the important part played by intuition and
deductive thought in the development of an
exact science. As soon as a science has emerged
from its initial stages, theoretical advances are
no longer achieved merely by a process of arrange-
ment. Guided by empirical data, the investigator
rather develops a system of thought which, in
148
EXPERIMENTAL CONTffiMATION 149
general, is built up logically from a small number
of fundamental assumptions, the so-called axioms.
We call such a system of thought a theory. The
theory finds the justification for its existence in
the fact that it correlates a large number of single
observations, and it is just here that the "truth"
of the theory hes.
Corresponding to the same complex of empirical
data, there may be several theories, which differ
from one another to a considerable extent. But
as regards the deductions from the theories which
are capable of being tested, the agreement be-
tween the theories may be so complete, that it
becomes difficult to fiind such deductions in which
the two theories differ from each other. As an
example, a case of general interest is available in
the province of biology, in the Darwinian theory
of the development of species by selection in
the struggle for existence, and in the theory of
development which is based on the hypothesis
of the hereditary transmission of acquired char-
acters.
We have another instance of far-reaching
agreement between the deductions from two
theories in Newtonian mechanics on the one hand,
and the general theory of relativity on the other.
This agreement goes so far, that up to the present
we have been able to find only a few deductions
from the general theory of relativity which are
150 APPENDIX in
capable of investigation, and to which the physics
of pre-relativity days does not also lead, and
this despite the profound difference in the funda-
mental assumptions of the two theories. In
what foUows, we shall again consider these im-
portant deductions, and we shall also discuss
the empirical evidence appertaining to them
which has hitherto been obtained.
(a) Motion of the Perihelion of Mercury
According to Newtonian mechanics and New-
ton's law of gravitation, a planet which is revolving
round the sim would describe an ellipse round the
latter, or, more correctly, round the common
centre of gravity of the sun and the planet. In
such a system, the svm, or the common centre of
gravity, Ues in one of the foci of the orbital ellipse
in such a manner that, in the course of a planet-
year, the distance sun-planet grows from a mini-
mxmi to a maximum, and then decreases again
to a rrii riiTmnn . If instead of Newton's law we
insert a somewhat different law of attraction into
the calculation, we find that, according to this
new law, the motion would still take place in such
a manner that the distance sim-planet exhibits
periodic variations; but in this case the angle
described by the line joining sun and planet
during such a period (from perihelion — closest
EXPERIMENTAL CONFIRMATION 151
proximity to the sun — to perihelion) would
differ from 360°. The line of the orbit would not
then be a closed one, but in the course of time
it would fiU up an annular part of the orbital
plane, viz. between the circle of least and the
circle of greatest distance of the planet from
the sun.
According also to the general theory of relativity,
which differs of course from the theory of Newton,
a small variation from the Newton-Kepler mo-
tion of a planet in its orbit should take place, and
in such a way, that the angle described by the
radius sun-planet between one perihehon and the
next should exceed that corresponding to one
complete revolution by an amount given by
{N.B. — One complete revolution corresponds
to the angle 2 tt in the absolute angular measure
customary in physics, and the above expression
gives the amount by which the radius sun-planet
exceeds this angle during the interval between
one perihehon and the next.) In this expression
a represents the major semi-axis of the eUipse,
e its eccentricity, c the velocity of Hght, and T
the period of revolution of the planet. Our
result may also be stated as follows: According
to the general theory of relativity, the major axis
of the ellipse rotates roimd the sun in the same
152 APPENDIX III
sense as the orbital motion of the planet. Theory
requires that this rotation shoiild amount to 43
seconds of arc per century for the planet Mercury,
but for the other planets of our solar system its
magnitude should be so small that it would
necessarily escape detection.^
In point of fact, astronomers have foimd that
the theory of Newton does not suffice to cal-
culate the observed motion of Mercury with an
exactness corresponding to that of the delicacy
of observation attainable at the present time.
After taking account of all the disturbing in-
fluences exerted on Mercury by the remaining
planets, it was foimd (Leverrier — 1859 — ^^d
Newcomb — 1895) that an vinexplained periheUal
movement of the orbit of Mercury remained over,
the amount of which does not differ sensibly from
the above-mentioned + 43 seconds of arc per
century. The uncertainty of the empirical result
amounts to a few seconds only.
(b) Deflection of Light Sy a
Gravitational Field
In Section XXII it has been aheady mentioned
that, according to the general theory of relativity,
a ray of light wiU experience a curvature of its
* Especially since the next planet Venus has an orbit that is
almost an exact circle, which makes it more difficult to locate the
perihelion with precision.
EXPERIMENTAL CONFIRMATION 153
path when passing through a gravitational field,
this curvature being similar to that experienced
by the path of a body which is projected through
a gravitational field. As a result of this theory,
we should expect that a ray of light which is
passing close to a heavenly body would be deviated
towards the latter. For a ray of fight which
passes the sun at a distance of A sun-radii from
its centre, the angle of deflection (a) should
amoimt to
I • 7 seconds of arc
a ^
It may be added that, according to the theory,
half of this deflection is produced by the New-
tonian field of attraction of the
sun, and the other half by the }
geometrical modification ("curva- i •
ture ") of space caused by the sun. /
This result admits of an experi- / /
mental test by means of the /-^^''
photographic registration of stars 7/
during a total eclipse of the sxm. ^//Dj,
The only reason why we must /'
wait for a total ecHpse is because ^
at every other time the atmos- Ym. $.
phere is so strongly illvmiinated
by the light from the sim that the stars situated
near the sun's disc are invisible. The predicted
effect can be seen clearly from the accompanying
154 APPENDIX III
diagram. If the sun (5) were not present, a star
which is practically iir&nitely distant would be
seen in the direction A, as observed from the
earth. But as a consequence of the deflection of
light from the star by the sim, the star will be
seen in the direction A, i.e. at a somewhat greater
distance from the centre of the sim than corre-
sponds to its real position.
In practice, the question is tested in the fol-
lowing way. The stars in the neighbourhood of
the sun are photographed during a solar ecUpse.
In addition, a second photograph of the same
stars is taken when the sun is situated at another
position in the sky, i.e. a few months earlier or
later. As compared with the standard photograph,
the positions of the stars on the eclipse-photograph
ought to appear displaced radially outwards
(away from the centre of the sun) by an amount
corresponding to the angle a.
We are indebted to the Royal Society and to
the Royal Astronomical Society for the investiga-
tion of this important deduction. Undaunted
by the war and by difficulties of both a material
and a psychological nature aroused by the war,
these societies equipped two expeditions — to
Sobral (Brazil) and to the island of Principe
(West Africa) — and sent several of Britain's
most celebrated astronomers (Eddington, Cotting-
ham, Crommelin, Davidson), in order to obtain
i EXPERIMENTAL CONFIRMATION 155
photographs of the solar eclipse of 29th May,
1919. The relative discrepancies to be expected
between the stellar photographs obtained during
the eclipse and the comparison photographs
amounted to a few hundredths of a millimetre
only. Thus great accuracy was necessary in
making the adjustments required for the taking
of the photographs, and in their subsequent
measurement.
The results of the measurements confirmed the
theory in a thoroughly satisfactory manner. The
rectangular components of the observed and of
the calculated deviations of the stars (in seconds
of arc) are set forth in the following table of
results:
Number of the
First Co-ordinate.
Second Co-ordinate.
Star.
'
^
Observed Calculated. Observed.
Calculated.
II
— 0-19
— 0-22
-f o-i6
+ 0-02
5 •
+ 0-29
+ 0-31
— 0-46
-0-43
4 .
+ 0-II
+ O-IO
+ 0-83
+ 0-74
3 •
+ 0-20
+ 0-I2
+ I-OO
+ 0-87
6 .
+ 0-I0
+ 0-04
+ 0-57
-t- 0-40
10
-o-o8
+ 0-09
+ 0-3S
+ 0-32
2
+ 0-9S
+ 0-85
— 0-27
— 0-09
(c) Displacement or Spectral Lines
TOWABDS the ReD
In Section XXIII it has been shown that in a
system K' which is in rotation with regard to a
Galileian system K, clocks of identical construe-
156 APPENDIX III
tion, and which are considered at rest with respect
to the rotating reference-body, go at rates which
are dependent on the positions of the clocks. We
shall now examine this dependence quantitatively.
A clock, which is situated at a distance r from the^
centre of the disc, has a velocity relative to K
which is given by
where ci> represents the angular velocity of rotation
of the disc Z' with respect XaK. If I'o represents the
number of ticks of the clock per imit time ("rate"
of the clock) relative to K when the clock is at
rest, then the "rate" of the clock iy) when it is
moving relative to K with a velocity v, but at rest
with respect to the disc, will, in accordance with
Section XII, be given by
or with sufficient accuracy by
This expression may also be stated in the fol-
lowing form:
If we represent the difference of potential of the
centrifugal force between the position of the clock
and the centre of the disc by <^, i.e. the work,
EXPERIMENTAL CONFIRMATIOM 167
considered negatively, which must be performed
on the unit of mass against the centrifugal force
in order to transport it from the position of the
clock on the rotating disc to the centre of the disc,
then we have
<!> = -—■
From this it follows that
In the first place, we see from this expression that
two clocks of identical construction will go at
different rates when situated at different distances
from the centre of the disc. This result is also
valid from the standpoint of an observer who is
rotating with the disc.
Now, as judged from the disc, the latter is in a
gravitational field of potential <[>, hence the result
we have obtained will hold quite generally for
gravitational fields. Furthermore, we can regard
an atom which is emitting spectral lines as a
clock, so that the following statement will
hold:
An atom absorbs or emits light of a frequency
which is dependent on the potential of the gravita-
tional field in which it is situated.
The frequency of an atom situated on the
surface of a heavenly body will be somewhat
less than the frequency of an atom of the same
158 APPENDIX in
element which is situated in free space (or on the
surface of a smaller celestial body).
M
Now 4>= - K—, where K is Newton's constant of
gravitation, and M is the mass of the heavenly
body. Thus a displacement towards the red ought
to take place for spectral lines produced at the
surface of stars as compared with the spectral lines
of the same element produced at the surface of
the earth, the amoimt of this displacement being
Va-v KM
Vo ^ r
For the sun, the displacement towards the red
predicted by theory amotmts to about two mil-
lionths of the wave-length. A trustworthy cal-
culation is not possible in the case of the stars,
because in general neither the mass M nor the
radius r is known. ^
It is an open question whether or not this effect
exists, and at the present time astronomers are
working with great zeal towards the solution.
Owing to the smaUness of the effect in the case of
the sun, it is difficult to form an opinion as to its
existence. Whereas Grebe and Bachem (Bonn),
as a result of their own measurements and those
of Evershed and Schwarzschild on the cyanogen
bands, have placed the existence of the effect
almost beyond doubt, other investigators, par-
EXPERIMENTAL COlSnPIEMATION 159
ticularly St. John, have been led to the opposite
opinion in consequence of their measurements.
Mean displacements of lines towards the less
refrangible end of the spectrum are certainly
revealed by statistical investigations of the fixed
stars; but up to the present the examination of
the available data does not aUow of any definite
decision being arrived at, as to whether or not
these displacements are to be referred in reaUty
to the effect of gravitation. The results of ob-
servation have been collected together, and dis-
cussed in detail from the standpoint of the ques-
tion which has been engaging our attention here,
in a paper by E. Fretmdlich entitled "Zur Priifung
der aUgemeinen Relativitats-Theorie" (Die Na-
turwissenschaften, 1919, No. 35, p. 520: Julius
Springer, Berlin).
At aU events, a definite decision will be reached
during the next few years. If the displacement
of spectral lines towards the red by the gravita-
tional potential does not exist, then the general
theory of relativity will be untenable. On the
other hand, if the cause of the displacement of
spectral lines be definitely traced to the gravita-
tional potential, then the study of this displace-
ment will furnish us with important information
as to the mass of the heavenly bodies.
BIBLIOGRAPHY
WORKS IN ENGLISH ON EINSTEIN'S
THEORY
Introductory
The Foundations of Einstein's Theory of Gravitation: Erwin Freund-
lich (translation by H. L. Brose). Camb. Univ. Press, 1920.
Space and Time in Contemporary Physics: Moritz Schlick (trans-
lation by H. L. Brose). Clarendon Press, Oxford, 1920.
The Special Theory
The Principle of Relativity: E. Cunningham. Camb. Univ. Press.
Relativity and the Electron Theory: E. Cunningham, Monographs on
Physics. Longmans, Green & Co.
The Theory of Relativity: L. Silberstein. Macmillan & Co.
The Space-Time Manifold of Relativity: E. B. Wilson and G. N.
Lewis, Proc. Amer. Soc. Arts ir Science, vol. xlviii.. No. 11, 1912.
The Generai. Theory
Report on the Relativity Theory of Gravitation: A. S. Eddington.
Fleetway Press Ltd., Fleet Street, London.
On Einstein's Theory of Gravitation and its Astronomical Consequences:
W. de Sitter, M. N. Roy. Astron. Soc, Ixxvi. p. 699, 1916; Ixxvii.
p. IS5, 1916; kxviii. p. 3, 1917.
On Einstein's Theory of Gravitation: H. A. Lorentz, Proc. Amsterdam
Acad., vol. xix. p. 1341, 1917.
Space, Time and Gravitation: W. de Sitter: The Observatory, No. $05,
p. 4r2. Taylor & Francis, Fleet Street, London.
161
162 BIBLIOGRAPHY
The Total EcUpse of sgth May igig, and the Influence of Gravitation
on Light; A. S. Eddlngton, iMd., March, 1919.
Discussion on the Theory of Relativity: M. N. Roy. Astron. Sac, vol.
kxx., No. 2., p. 96, December 1919.
The Displacement of Spectrum Lines and the Equivalence Hypothesis:
W. G. Duffield, M. N. Roy. Astron. Soc, vol. Ixxx.; No. 3, p. 262,
1920.
Space, Time and Gravitation: A. S. Eddington. Camb. Univ. Press,
1920.
Also, Chapters in '
The Mathematical Theory of Electricity and Magnetism: J. H. Jeans
(4th edition). Camb. Univ. Press, 1920.
The Electron Theory of Matter: O. W. Richardson. Camb. Univ.
Press.
INDEX
INDEX
Aberration, S9
Absorption of energy, 54
Acceleration, 76, 78, 83
Action at a distance, 57
Addition of velocities, 19, 46
Adjacent points, 105
Aether, 62
drift, 62, 63
Arbitrary substitutions, 116
Astronomy, 8, 121
Astronomical day, is
Axioms, 2, 149
— truth of, 2
Bachem, 158
Basis of theory, 52
" Being," 78, 128
^-rays, 59
Biology, 149
Cartesian system of co-ordi-
nates, 7, 100, 147
Cathode rays, 59
Celestial mechanics, 125
Centrifugal force, 94, 156
Chest, 78
Classical mechanics, 9, 13, 16,
20, 36, 52, 84, 121, 123,
'150
— truth of, IS
Clocks, II, 28, 9S, 96, 112, 114,
117-120, 121, 13s, iss
— rate of, 156
Conception of mass, 54
— position, 6
Conservation of energy, 54, 121
— impulse, 121
— mass, 54, 56
Continuity, 113
Continumn, 65, 98
Continuum, two-dimensional,
112
— three-dimensional, 67
— four-dimensional, 106, 108,
109, 112, 147
— space-time, 93, 108-114
— Euclidean, 99, loi, 104, no
— non-Euclidean, 102, 107
Co-ordinate diflEerences, 109
— differentials, 109
— planes, 38
Cottingham, 154
Counter-point, 133
Co-variant, 51
Crommelin, 154
Curvature of light-rays, 124,
152
— space, 153
Curvilinear motion, 88
Cyanogen bands, 158
Darwinian theory, 149
Davidson, 154
Deductive thought, 148
Derivation of laws, 52
165
166
INDEX
De Sitter, 21
Displacement of spectral Unes,
124, iSS
Distance (line-interval), 3, 5,
8, 34, 3S. 99, 104, 129
— physical interpretation of, 5
— relativity of, 34
Doppler principle, 59
Double stars, 21
Eclipse of star, 21
Eddington, 124, 154
Electricity, 90
Electrodynamics, 15, 24, 48,
52, 90
Electromagnetic theory, 58
— waves, 75
Electron, 52, 60
— electrical masses of, 60
Electrostatics, go
Elliptical space, 133
Empirical laws, 148
Encounter (space-time coin-
cidence), 113
Equivalent, 16
Euclidean geometry, i, 2, 68,
97, loi, 104, 128, 129,
13s, 147
propositions of, 3, 8
— space, 68, 102, 147
Evershed, 158
Experience, S9, 70
Faraday, 56, 74
FitzGerald, 63
Fixed stars, 12
Fizeau, 46, 58, 61
Fizeau, experiment of, 46
Frequency of atom, 157
Galilei, 12
— transformation, 40, 43, 45,
SO, 61
GalHeian system of co-ordi-
nates, 13, 15, 17, 54, 93,
108, 116, 119
Gauss, 102, 103, 106
Gaussian co-ordinates, 103-
105, 112, ii4-ri8
General theory of relativity,
69-124, IIS
Geometrical ideas, 2, 3
— propositions, i
truth of, 2-4
Gravitation, 75, 82, 92, 121
Gravitational field, 75, 79, 87,
91, III, 116, 119, 120,
136
potential of, 157
— mass, 76, 81, 121
Grebe, 158
Group-density of stars, 126
Helmholtz, 128
Heuristic value of relativity,
SO
Induction, 148, 149
Inertia, 77
Inertial mass, 55, 76, 81, 120,
121
Instantaneous photograph
(snapshot), 141
Intensity of gravitational field,
127
INDEX
167
Intuition, 148
Ions, S3
Kepler, 152
Kinetic energy, S3i 121
Lattice, 128
Laws of Galilei-Newton, 15
Law of inertia, 12, 71, 72, 170
Laws of Nature, 70, 84, 118
Leverrier, 123, 152
Light-signal, 40, 139, 143
Light-stimulus, 40
Limiting velocity (c), 43, 44
Lines of force, 126
Lorentz, H. A., 24, 48, 52, 58,
59-63
— transformation, 39, 46, 50,
108, 116, 139, 143, 144,
146
(generalised), 144
Mach, E., 86
Magnetic field, 74
Manifold (.see Continuum)
Mass of heavenly bodies, 159
Matter, 120
Maxwell, 49, 52, 56-59, 61
— fundamental equations, 56,
90
Measurement of length, loi
Measuring-rod, s, 6, 34, 95,
96, 112, 119, 121, 132,
13s, 141
Mercury, 123, 152
— orbit of, 123, 152
Michelson, 62-64
Minkowski, 65-68, 108, 147
Morley, 63, 64
Motion, 16, 70
— of heavenly bodies, 16, 17,
52, 122, 13s
Newcomb, 152
Newton, 12, 86, 122, 126, 150
Newton's constant of gravita-
tion, 158
— law of gravitation, 57, 94,
127,149
— law of motion, 76
Non-Euclidean geometry, 128
Non-GalUeian reference-
bodies, 117
Non-uniform motion, 72
Optics, IS, 24, 52
Organ-pipe, note of, 17
Parabola, 9, 10
Path-curve, 10
PeriheUon of Mercury, iso-
'152
Physics, 8
— of measurement, 7
Place specification, 6
Plane, i, 128, 129
Poincare, 128
Point, I
Point-mass, energy of, 54
Position, 9
Principle of relativity, 15-17,
23> 24, 70
Processes of Nature, 50
Propagation of light, 21, 23,
24, 36, 108, 143
in liquid, 47
in gravitational fields, 88
168
INDEX
Quasi-Euclidean universe, 136
Quasi-spherical universe, 136
Radiation, 55
Radioactive substances, 59
Reference-body, $, 7, 8-11,
22, 28, 31, 32, 44, 70
rotating, 94
— mollusk, 118-120
Relative position, 3
— velocity, 141
Rest, 17
Riemann, 102, 128, 132
Rotation, 95, 147
Schwarzschild, 158
Seconds-clock, 44
Seeliger, 125, 127
Simultaneity, 26, 29-32, 96
— relativity of, 31
Size-relations, 107
Solar eclipse, 89, 153, 155
Space, 9, 62, 65, 125
— conception of, 24^
Space co-ordinates, 66, 96, 118
Space-interval, 36, 67
— point, 118
Space, two-dimensional, 128
— three-dimensional, 147
Special theory of Relativity,
1-68, 24
Spherical surface, 129
— space, 132, 133
St. John, 159
Stellar universe, 126
— photographs, 153
Straight line, 1-3, 9, 10, 97,
los, 129
Sjrstem of co-ordinates, Si 10,
II
Terrestrial space, 18
Theory, 148
— truth of, 149
Three-dimensional, 65
Time, conception of, 24, 61,
125
— co-ordinate, 66, 118
— in Physics, 26, 117, 146
— of an event, 28, 32
interval, 36, 67
Trajectory, 10
" Truth," 2
Uniform translation, 14, 69
Universe (World) , structure of,
128, 13s
— circumference of, 131
Universe, elliptical, 133, 136
— Euclidean, 130, 132
— space expanse (radius) of,
137
— spherical, 132, 136
Value of ir, 97, 130
Velocity of light, 11, 21, 22,
89. 143
Venus, 152
Weight (heaviness), 77
World, 65, 66, 130, 147
World-point, 147
radius, 133
sphere, 130, 131
Zeeman, 48
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